That ends our live coverage of the Germanwings plane crash for the day, after we learned that the aircraft appears to have been deliberately flown into the mountain by the co-pilot.
You can read our main story here and check back for more updates on the BBC news website later, with continued coverage on the BBC News channel.
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On a day of dramatic developments, here's a reminder of everything we know so far about the Germanwings crash in the French Alps that left 150 people dead.
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German investigators have been removing boxes of documents and other objects from the apartment of Andreas Lubitz in Duesseldorf.
ReutersCopyright: Reuters
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APCopyright: AP
Search and recovery teams have again spent a day in incredibly difficult conditions in the French Alps. The site of the crash is inaccessible by road and can only be reached from the air, making the salvage operation all the more challenging.
Darkness has now fallen on the area but at first light, they will be back again to continue their grim task.
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You can watch the video of rescuer Jean Sebastien Beaud's interview here.
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The US does not believe that there is a terrorism connection in the Germanwings crash. White House spokesman Josh Earnest said on Thursday that "based on what we know, there is not a nexus to terrorism."
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Jean Sebastien Beaud is a member of the mountain police and was one of the first on the scene after the Germanwings plane crashed on Tuesday. He and two colleagues were winched into the ravine just 30 minutes after the plane came down.
Mr Beaud - who recovered the black box from the plane - told the BBC: "As a rescuer, I go on many accidents and see a lot of ugly things, but never anything of this scale, never.
"The scene was surreal... and I knew from the first moment there would be no survivors."
BBCCopyright: BBC
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A single candle has been lit outside the home of the family of co-pilot Andreas Lubitz in Montabaur.
APCopyright: AP
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More than 100 relatives and friends of the victims have arrived in the small village of Seyne-les-Alpes, close to the site of the disaster.
But why visit the crash site? The BBC's William Kremer looks into the reasons behind their trip here.
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The BBC's Clive Myrie says that Andreas Lubitz's parents were among those who travelled to Seyne-les-Alpes to mourn those killed in the crash. Following the revelations from the cockpit voice recorder "they have now been separated from the rest of the group", he added.
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Sandrine Boisse, a tourism official from the ski resort of Pra Loup who believes she heard the moment of collision, told the BBC that "you feel like you're in a movie".
She added that it felt surreal to have "all the light of the world turned on your valley".
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Philip Baum, editor of Aviation Security International magazine, told BBC Radio 5 live that changes could be made to reduce the likelihood of a similar incident happening in the future.
He said: "I think that far too many pilots actually do leave the flight deck during short flights. I mean obviously on long haul flights, they do need to be able to leave the flight deck.
"Although there is an argument to say that one should actually have two flight deck doors - as El Al the Israeli airline has - so that a pilot can leave the flight deck, go to the toilet, but still remain behind a secondary door."
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Flags representing the nationalities of some of the victims were held up as their friends and relatives gathered for a ceremony in a field in Seyne-les-Alpes, close to the crash site.
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Montabaur Mayor Edmund Schaaf has said that the town's "deepest sympathies" go out to the victims' relatives, adding: "We are saying this regardless of the circumstances that led to this event."
He confirmed that co-pilot Andreas Lubitz came from Montabaur and that his family live in the town.
But he added: "Concerning the accusations that he deliberately crashed the plane, we cannot give, and do not want to give, any statement."
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Norwegian Air Shuttle and Air Canada had announced similar plans earlier on Thursday.
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British carrier Easyjet will now require two crew members to be in the cockpit at all times. The move comes after the UK's Civil Aviation Authority told UK airlines to review their procedures. You can read more about this on the BBC website here.
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The two homes of co-pilot Andreas Lubitz - in Duesseldorf and Montabaur - have been searched by investigators, according to the agency AFP.
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The search of the crash site in the Alps is ongoing and there are reports that bodies are now being recovered and airlifted from the area.
ReutersCopyright: Reuters
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The US State Department has named the third American passenger on board the Germanwings flight as Robert Oliver, Reuters has reported. The department is still investigating whether there were further US citizens involved.
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One of Andreas Lubitz's neighbours in Montabaur, Wolfgang Michaelis, tells the BBC that the co-pilot's actions were incomprehensible. "It's a terrible situation because the family has lost their son and because of all those innocent people who died, and I can't understand that someone, as it appears, commits suicide... that he then kills innocent people along with himself," Mr Michaelis says. "There is nothing to understand. This is particularly terrible."
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Germanwings has said in a statement that its employees are "stunned" over the revelation that the plane was brought down deliberately.
"We would have never imagined that such a tragedy could happen in our company," it added.
The company described the incident as a "tragic individual case" and said that it still had complete confidence in its pilots.
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The local government in Duesseldorf says the most recent regular security check on Andreas Lubitz, the Germanwings co-pilot, was conducted on 27 January and found nothing unusual, according to the Associated Press. The government says it also consulted the authorities in Bremen, where he went to flight school, and Rhineland-Palatinate state, where his hometown of Montabaur is located. The checks look for any criminal record or links to extremists.
'Mass murder'
James Reynolds
BBC News, Seyne-Les-Alpes
Marseille's prosecutor Brice Robin entered the room with no sign that he had an astonishing conclusion to reveal. He told reporters he had examined the voice recordings of the last 30 minutes of the flight. For the first 20 minutes, Mr Robin said, the captain and co-pilot spoke to each other normally - even cheerfully. Then the captain got up - apparently to use the toilet - and the cockpit door locked behind him. The co-pilot Andreas Lubitz was left alone in control of the plane. He soon pressed a button for the aircraft to descend. The prosecutor said he believed that this was a deliberate act.
AFPCopyright: AFP
The recordings picked up every detail of the co-pilot's actions - suggesting that he was conscious and alert. "We could hear him breathing. He breathed normally. He didn't utter a single word the minute the pilot left the cabin," Mr Robin said. The plane started to descend. The captain knocked on the cockpit door and used the intercom. But his co-pilot didn't respond. Trapped outside his own cockpit, the captain became increasingly desperate. "The alarms went off to alert the crew to the proximity of the ground and at that moment, we could hear a violent knocking on a door, someone was trying to force the door. It was a strong door, it was reinforced, in line with international norms, to protect against acts of terrorism." That meant there was no way in - and no way to stop the co-pilot. Andreas Lubitz calmly flew himself, his captain and their 148 crew and passengers straight into the mountainside.
ReutersCopyright: Reuters
"I think that the victims did not realise what was happening until the last moment, the very last moment. Because in the recording, we could only hear the cries at the last moment, just before the impact with the ground." The prosecutor described their deaths as sudden and immediate. Brice Robin said he believed that Andreas Lubitz deliberately crashed the plane. If that turns out to be true, the final 10 minutes of the 28-year-old co-pilot's life were a sustained act of mass murder.
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Twenty-eight-year-old co-pilot Andreas Lubitz is believed to have locked the doors of the cockpit and sent the Germanwings plane into descent with 150 people on board.
A German foreign ministry spokesman says that 75 German nationals are now believed to have been killed in the crash, based on current information, according to Reuters. On Wednesday, Germanwings had put the number of German victims at 72.
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A German state prosecutor has confirmed that police are searching properties in Duesseldorf and other locations for documents and evidence relating to the crash, Reuters news agency reports. Christoph Kumpa said it would take a while to assess the findings.
Andreas Lubitz lived with his parents in the town of Montabaur but also kept a flat in Duesseldorf.
APCopyright: AP
German police were pictured outside what was believed to be Andreas Lubitz's flat in DuesseldorfImage caption: German police were pictured outside what was believed to be Andreas Lubitz's flat in Duesseldorf
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The US FBI has offered to help the French authorities with the air crash investigation. "We stand ready to fulfil any requests for information," FBI spokesman Paul Bresson told the Associated Press.
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James Longman
BBC News
tweets: "A map circling #GermanWingsCrash exclusion zone from helicopter high over the #Alps @BBCNews"
James LongmanCopyright: James Longman
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Matthias Gebauer
Der Spiegel magazine
tweets: "Looking for a motive for the deadly descent: Police begin search warrant at #4U9525 co-driver Andreas L. Montabaur"
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Our correspondent Jenny Hill has been speaking to residents near the family home of Andreas Lubitz in Montabaur, shown below.
She reports: "'It's just horrible,' one neighbour told me as she held her little boy. She didn't know the family well at all. But she worries now that Montabaur will always be remembered for this.
"A little while ago two people strode up to the front door escorted by police officers. A woman, maybe in her 30s, carrying a notebook and a slightly older man were let into the house and the heavy brown door slammed shut again."
ReutersCopyright: Reuters
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Families of some of the victims have now arrived in Seyne-les-Alpes, near the crash site. The local bishop is shown here preparing to greet and support those relatives.
AFP/GettyCopyright: AFP/Getty
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BBC correspondent Jenny Hill is in Montabaur, where Germanwings co-pilot Andreas Lubitz is thought to have lived.
She says: "It's an unremarkable house in a small quiet housing estate. But this is where Andreas Lubitz is believed to have lived with his parents - and the house with grey walls and dormer windows is now the focus of the world's attention as investigators try to establish why he appears to have deliberately crashed the plane."
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She said today's news was "an additional strain on the families and in this hour full of suffering, these days full of suffering, our thoughts are especially with them".
BBCCopyright: BBC
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Mrs Merkel said that Germany would do all it could to support the investigation into the crash.
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German Chancellor Angela Merkel said she has been deeply shocked by the latest developments. "It is difficult to measure the suffering that this catastrophe has brought to so many families," she tells a news conference in Berlin. "Today, we now have received news that this tragedy has been given a new, immeasurably incomprehensible dimension."
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How are cockpit doors locked? BBC Magazine takes a look at how aircraft security systems work here.
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A 30-minute recording obtained from the cockpit voice recorder has provided the clearest indication yet as to what might have happened on board. Take a look at our analysis of the flight's final half hour.
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A coach transporting relatives of some of the victims has arrived in Seyne-les-Alpes, near the crash site in the French Alps.
ReutersCopyright: Reuters
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Lufthansa CEO Carsten Spohr has described the entry mechanism of the cockpit door.
"There is another code you can enter at the door which will lead to a bell ringing and if nobody reacts, the door will open automatically and allow entry," he said.
But he added that this can be blocked from the cockpit by pushing a switch that will lock the door for five minutes.
"Either the captain didn't enter the code correctly - which we find unlikely because everybody involved knows this code by heart - or the colleague had entered the code and the co-pilot prevented the door from opening by pushing the lock switch," said Mr Spohr.
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Low-cost airline Norwegian Air Shuttle will now require two people to be in the cockpit at all times for safety reasons. "When one person leaves the cockpit, two people will now have to be there," its director of operations is quoted as saying by AFP news agency.
French prosecutors say the pilot of the Germanwings plane that crashed was locked out of the cockpit by his co-pilot, who then "voluntarily" sent the plane into a descent.
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A BBC producer says she has seen police officers entering the house. It is in a well-manicured, quiet cul-de-sac on the outskirts of Montabaur, she adds.
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German police have been keeping the media away from the door of Andreas Lubitz's home in the town of Montabaur, about 60km (37 miles) north-west of Frankfurt. Neighbours have refused to comment.
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Earlier, Germany's Interior Minister Thomas de Maiziere told journalists that Andreas Lubitz was not known to the country's police or intelligence agencies. Neither, he added, had the co-pilot been flagged in any Lufthansa security checks.
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Lufthansa CEO Carsten Spohr told a news conference in Cologne that Andreas Lubitz began his training in 2008. "After his training there was an 11-month waiting period - this is not unusual with us - and since 2013 he was working as co-pilot on the Airbus A320."
He added: "I want to mention that there was a longer break in his training six years ago and after his suitability was assessed again he resumed his training. Subsequently, he passed all medical checks as well as all aviation training. He was 100% fit to fly without any restrictions or conditions."
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Members of the LSC Westerwald flying club earlier said Mr Lubitz had gained his glider pilot's licence as a teenager and that he had shown no signs of depression when he renewed that licence in the autumn.
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The chairman of Mr Lubitz's flying club, Klaus Radke, told AP that he rejected the French prosecutor's assessment that he had deliberately crashed the plane into the Alps. "I don't see how anyone can draw such conclusions before the investigation is completed."
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Members of Andreas Lubitz's flying club in Montabaur, Germany, say the 28 year old appeared to be content with his job. "He was happy he had the job with Germanwings and he was doing well," longtime LSC Westerwald member Peter Ruecker told the Associated Press. "He was very happy, he gave off a good feeling."
AFPCopyright: AFP
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Mr Spohr says: "No matter your safety regulations, no matter how high you set the bar, and we have incredibly high standards, there is no way to rule out such an event."
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Lufthansa CEO Carsten Spohr tells a news conference in Cologne: "We choose our staff very, very carefully."
AFPCopyright: AFP
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Mr Spohr says there is no indication about what the co-pilot's motive was.
FacebookCopyright: Facebook
Andreas Lubitz - pictured here on his social media profile - is not known to have any links to terrorismImage caption: Andreas Lubitz - pictured here on his social media profile - is not known to have any links to terrorism
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Lufthansa CEO Carsten Spohr says: "In our worst nightmares we couldn't have imagined that such a tragedy could happen within our company."
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Germany's Transport Minister, Alexander Dobrindt, said the French authorities' assessment that the plane's descent into the Alps was initiated deliberately was "according to our experts, plausible". He added: "We are in very close contact with Lufthansa regarding the conclusions that need to be drawn from this tragedy. One can only say that what the French prosecution described in great detail today is indeed more than harrowing."
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Lufthansa chairman and CEO Carsten Spohr tells a news conference in Cologne that staff at the airline, and those at its subsidiary Germanwings, are "absolutely speechless" and "deeply shocked" at the news of the co-pilot's actions. "I would not have been able to imagine that the situation could get even worse," he adds.
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Just to recap on what we have learnt in the last few hours. Officials in France say the co-pilot of the Germanwings Airbus A320 - which crashed into the French Alps on Tuesday, killing all 150 people on board - appears to have deliberately brought the plane down. Marseille prosecutor Brice Robin said Andreas Lubitz, 28, "voluntarily" sent the plane into a descent, while the pilot was locked out of the cockpit. But he said it did not appear to be a terrorist act.
EPACopyright: EPA
A four-day-old photograph of the cockpit of the crashed Germanwings Airbus A320 was published on ThursdayImage caption: A four-day-old photograph of the cockpit of the crashed Germanwings Airbus A320 was published on Thursday
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German Transport Minister Alexander Dobrindt says the emerging details of what happened on flight 4U 9525 reveal a situation that is "truly more than shocking".
AFPCopyright: AFP
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Prime Minister Mariano Rajoy of Spain says via Twitter that he is "deeply shaken by the latest information given by the investigators". He added: "Once again, our heartfelt affection to the families". There were 50 Spaniards on board the flight from Barcelona to Duesseldorf.
EPACopyright: EPA
Prime Minister Mariano Rajoy briefed King Felipe VI of Spain on the latest developments on ThursdayImage caption: Prime Minister Mariano Rajoy briefed King Felipe VI of Spain on the latest developments on Thursday
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The captain of Airbus A320, who has not been identified, had more than 10 years of experience with German flag carrier Lufthansa and its subsidiaries, including Germanwings. He had more than 6,000 hours of flight time, most of them on Airbus planes.
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The AFP news agency reports that the co-pilot, Andreas Lubitz, was registered as a member of a private flying club in Germany, LSC Westerwald, and was a runner who took part in local races, according to public records. This is a screengrab of what is believed to be his Facebook profile.
ReutersCopyright: Reuters
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On its Twitter feed, Germanwings says: "We are shaken by the upsetting statements of the French authorities. Our thoughts and prayers continue to be with the families and friends of the victims." The airline will hold a news conference at 14:30 German time (13:30 GMT).
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David Gleave, an air accident investigator whose speciality is pilot suicides, says the aviation industry needs to start looking at the psychological profiles of pilots. "We may find that this pilot had various stresses that may have led to some form of relatively irrational behaviour. It could be a vast amount of debt taken out for pilot training, it could be family matters, it could be religious matters. There's all sorts of areas that the psychological specialist will start to look at during the investigation but I certainly think that it's going to be reviewed fairly quickly."
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French prosecutor Brice Robin said the passengers on flight 4U 9525 only realised "at the last moment" they were about to crash, because "we only hear the screams on the last moments of the recording".
AFPCopyright: AFP
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Richard Westcott
BBC Transport Correspondent
This kind of event is rare but it has happened before, although the reasons vary widely. After 9/11 they made cockpits impregnable. It keeps the terrorists out, but in the end it also allows someone to keep their colleagues out too. Airlines have to make a call. Which is the bigger threat - terrorism or suicide?
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Richard Westcott
BBC Transport Correspondent
The focus now moves from the mechanics to the man flying the plane. An accident expert has told me the investigators will pore over the co-pilot's background and that of his family too. Did he owe money? Was there a grudge? They'll look at his religion, whether he was in trouble with the law, whether he had a stable love life.
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Sean Maffett, an aviation analyst, says a change in staffing levels might help prevent such incidents in future. "One possible answer to this is to insist all flights have three pilots on board, so there are always two of them on the flight deck. But airlines are run by accountants - that's horrendously much more expensive. It's a problem that is not going to be resolved straight away."
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German police have been deployed outside what is thought to be Andreas Lubitz's home in Montabaur, in west Germany.
APCopyright: AP
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German Interior Minister Thomas de Maiziere says there are no indications that the co-pilot, Andreas Lubitz, had "any kind of terrorist background", the Reuters news agency reports.
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Aviation analyst Sean Maffett says there is no requirement for psychiatric tests on pilots. "There have been calls for them in the past, and the calls will be renewed for that to happen," he tells the BBC. "I know that in the pilot fraternity there is considerable worry that that is going to happen, because it'll do nothing for most people but make their lives much more difficult."
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According to the audio recording from the flight recorder, the captain is heard asking the co-pilot to take over controls of the plane. Then there is the sound of a chair being pushed back and a door being closed. Moments later the captain is heard knocking on the door and asking to be let in, but receives no answer from the co-pilot.
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Germanwings flight 4U 9525 crashed on Tuesday into the French Alps en route from Barcelona to Duesseldorf. The German authorities are now taking charge of the investigation into co-pilot Andreas Lubitz, according to French prosecutors.
AFPCopyright: AFP
The plane came down near the town of Seyne-les-Alpes, where relatives of those killed have gatheredImage caption: The plane came down near the town of Seyne-les-Alpes, where relatives of those killed have gathered
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Mr Robin said pounding on the cockpit door could be heard during the final minutes of the flight of the Germanwings airliner, and that alarms sounded. He also said the co-pilot's breathing was normal throughout the final minutes.
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During the news conference, Mr Robin refused to give details on Andreas Lubitz's religion, saying: "I don't think it's necessarily what we should be looking for."
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Information taken from the black box cockpit voice recorder indicated that the co-pilot Andreas Lubitz did not say a word once the captain left the cockpit, Mr Robin said. "It was absolute silence in the cockpit."
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According to Lufthansa, which owns Germanwings, the co-pilot joined the airline in September 2013, immediately after training, and had flown 630 hours.
AFPCopyright: AFP
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Brice Robin said the responses of the co-pilot, named as Andreas Lubitz, 28, were initially courteous but became "curt" when the captain began to give a mid-flight briefing, about the planned landing of the Germanwings flight. The plane's eventual crash in the French alps killed 150 people.
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Mr Robin also said the co-pilot "pushed the button to trigger the aircraft to lose altitude. He operated this button for a reason we don't know yet, but it appears that the reason was to destroy this plane".
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Brice Robin, the state prosecutor in Marseille, told a news conference the co-pilot was alone at the controls when the plane came down. Analysis of the flight recorder showed the pilot had left the cockpit and was then calling on the co-pilot to let him back in, but there was no reply, he added. The co-pilot was heard breathing - showing that he was still alive at the time of the impact. "The most plausible interpretation is that the co-pilot through a voluntary act had refused to open the cabin door to let the captain in," Mr Robin said.
APCopyright: AP
Among the victims were 16 teenagers and two teachers from the Joseph-Koenig Gymnasium in Haltern am See, GermanyImage caption: Among the victims were 16 teenagers and two teachers from the Joseph-Koenig Gymnasium in Haltern am See, Germany
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Welcome to the BBC's live coverage of reaction to the announcement by prosecutors in the French city of Marseille that all the evidence from the cockpit voice recorder from the crashed Germanwings airliner suggests the co-pilot was responsible for the disaster, purposely steering the plane into the Alps.
Live Reporting
Camila Ruz and Lauren Turner
All times stated are UK
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APCopyright: AP German police were pictured outside what was believed to be Andreas Lubitz's flat in DuesseldorfImage caption: German police were pictured outside what was believed to be Andreas Lubitz's flat in Duesseldorf 
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FacebookCopyright: Facebook Andreas Lubitz - pictured here on his social media profile - is not known to have any links to terrorismImage caption: Andreas Lubitz - pictured here on his social media profile - is not known to have any links to terrorism 
EPACopyright: EPA A four-day-old photograph of the cockpit of the crashed Germanwings Airbus A320 was published on ThursdayImage caption: A four-day-old photograph of the cockpit of the crashed Germanwings Airbus A320 was published on Thursday 
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EPACopyright: EPA Prime Minister Mariano Rajoy briefed King Felipe VI of Spain on the latest developments on ThursdayImage caption: Prime Minister Mariano Rajoy briefed King Felipe VI of Spain on the latest developments on Thursday 
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AFPCopyright: AFP The plane came down near the town of Seyne-les-Alpes, where relatives of those killed have gatheredImage caption: The plane came down near the town of Seyne-les-Alpes, where relatives of those killed have gathered 
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APCopyright: AP Among the victims were 16 teenagers and two teachers from the Joseph-Koenig Gymnasium in Haltern am See, GermanyImage caption: Among the victims were 16 teenagers and two teachers from the Joseph-Koenig Gymnasium in Haltern am See, Germany
Latest PostPost update
That ends our live coverage of the Germanwings plane crash for the day, after we learned that the aircraft appears to have been deliberately flown into the mountain by the co-pilot.
You can read our main story here and check back for more updates on the BBC news website later, with continued coverage on the BBC News channel.
Post update
On a day of dramatic developments, here's a reminder of everything we know so far about the Germanwings crash in the French Alps that left 150 people dead.
Post update
German investigators have been removing boxes of documents and other objects from the apartment of Andreas Lubitz in Duesseldorf.
Post update
Search and recovery teams have again spent a day in incredibly difficult conditions in the French Alps. The site of the crash is inaccessible by road and can only be reached from the air, making the salvage operation all the more challenging.
Darkness has now fallen on the area but at first light, they will be back again to continue their grim task.
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You can watch the video of rescuer Jean Sebastien Beaud's interview here.
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The US does not believe that there is a terrorism connection in the Germanwings crash. White House spokesman Josh Earnest said on Thursday that "based on what we know, there is not a nexus to terrorism."
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Jean Sebastien Beaud is a member of the mountain police and was one of the first on the scene after the Germanwings plane crashed on Tuesday. He and two colleagues were winched into the ravine just 30 minutes after the plane came down.
Mr Beaud - who recovered the black box from the plane - told the BBC: "As a rescuer, I go on many accidents and see a lot of ugly things, but never anything of this scale, never.
"The scene was surreal... and I knew from the first moment there would be no survivors."
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A single candle has been lit outside the home of the family of co-pilot Andreas Lubitz in Montabaur.
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More than 100 relatives and friends of the victims have arrived in the small village of Seyne-les-Alpes, close to the site of the disaster.
But why visit the crash site? The BBC's William Kremer looks into the reasons behind their trip here.
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The BBC's Clive Myrie says that Andreas Lubitz's parents were among those who travelled to Seyne-les-Alpes to mourn those killed in the crash. Following the revelations from the cockpit voice recorder "they have now been separated from the rest of the group", he added.
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Sandrine Boisse, a tourism official from the ski resort of Pra Loup who believes she heard the moment of collision, told the BBC that "you feel like you're in a movie".
She added that it felt surreal to have "all the light of the world turned on your valley".
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Philip Baum, editor of Aviation Security International magazine, told BBC Radio 5 live that changes could be made to reduce the likelihood of a similar incident happening in the future.
He said: "I think that far too many pilots actually do leave the flight deck during short flights. I mean obviously on long haul flights, they do need to be able to leave the flight deck.
"Although there is an argument to say that one should actually have two flight deck doors - as El Al the Israeli airline has - so that a pilot can leave the flight deck, go to the toilet, but still remain behind a secondary door."
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Flags representing the nationalities of some of the victims were held up as their friends and relatives gathered for a ceremony in a field in Seyne-les-Alpes, close to the crash site.
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Montabaur Mayor Edmund Schaaf has said that the town's "deepest sympathies" go out to the victims' relatives, adding: "We are saying this regardless of the circumstances that led to this event."
He confirmed that co-pilot Andreas Lubitz came from Montabaur and that his family live in the town.
But he added: "Concerning the accusations that he deliberately crashed the plane, we cannot give, and do not want to give, any statement."
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Norwegian Air Shuttle and Air Canada had announced similar plans earlier on Thursday.
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British carrier Easyjet will now require two crew members to be in the cockpit at all times. The move comes after the UK's Civil Aviation Authority told UK airlines to review their procedures. You can read more about this on the BBC website here.
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The two homes of co-pilot Andreas Lubitz - in Duesseldorf and Montabaur - have been searched by investigators, according to the agency AFP.
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The search of the crash site in the Alps is ongoing and there are reports that bodies are now being recovered and airlifted from the area.
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The US State Department has named the third American passenger on board the Germanwings flight as Robert Oliver, Reuters has reported. The department is still investigating whether there were further US citizens involved.
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One of Andreas Lubitz's neighbours in Montabaur, Wolfgang Michaelis, tells the BBC that the co-pilot's actions were incomprehensible. "It's a terrible situation because the family has lost their son and because of all those innocent people who died, and I can't understand that someone, as it appears, commits suicide... that he then kills innocent people along with himself," Mr Michaelis says. "There is nothing to understand. This is particularly terrible."
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Germanwings has said in a statement that its employees are "stunned" over the revelation that the plane was brought down deliberately.
"We would have never imagined that such a tragedy could happen in our company," it added.
The company described the incident as a "tragic individual case" and said that it still had complete confidence in its pilots.
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The local government in Duesseldorf says the most recent regular security check on Andreas Lubitz, the Germanwings co-pilot, was conducted on 27 January and found nothing unusual, according to the Associated Press. The government says it also consulted the authorities in Bremen, where he went to flight school, and Rhineland-Palatinate state, where his hometown of Montabaur is located. The checks look for any criminal record or links to extremists.
'Mass murder'
James Reynolds
BBC News, Seyne-Les-Alpes
Marseille's prosecutor Brice Robin entered the room with no sign that he had an astonishing conclusion to reveal. He told reporters he had examined the voice recordings of the last 30 minutes of the flight. For the first 20 minutes, Mr Robin said, the captain and co-pilot spoke to each other normally - even cheerfully. Then the captain got up - apparently to use the toilet - and the cockpit door locked behind him. The co-pilot Andreas Lubitz was left alone in control of the plane. He soon pressed a button for the aircraft to descend. The prosecutor said he believed that this was a deliberate act.
The recordings picked up every detail of the co-pilot's actions - suggesting that he was conscious and alert. "We could hear him breathing. He breathed normally. He didn't utter a single word the minute the pilot left the cabin," Mr Robin said. The plane started to descend. The captain knocked on the cockpit door and used the intercom. But his co-pilot didn't respond. Trapped outside his own cockpit, the captain became increasingly desperate. "The alarms went off to alert the crew to the proximity of the ground and at that moment, we could hear a violent knocking on a door, someone was trying to force the door. It was a strong door, it was reinforced, in line with international norms, to protect against acts of terrorism." That meant there was no way in - and no way to stop the co-pilot. Andreas Lubitz calmly flew himself, his captain and their 148 crew and passengers straight into the mountainside.
"I think that the victims did not realise what was happening until the last moment, the very last moment. Because in the recording, we could only hear the cries at the last moment, just before the impact with the ground." The prosecutor described their deaths as sudden and immediate. Brice Robin said he believed that Andreas Lubitz deliberately crashed the plane. If that turns out to be true, the final 10 minutes of the 28-year-old co-pilot's life were a sustained act of mass murder.
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Twenty-eight-year-old co-pilot Andreas Lubitz is believed to have locked the doors of the cockpit and sent the Germanwings plane into descent with 150 people on board.
But who was he? Here is everything we know about Andreas Lubitz
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A German foreign ministry spokesman says that 75 German nationals are now believed to have been killed in the crash, based on current information, according to Reuters. On Wednesday, Germanwings had put the number of German victims at 72.
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A German state prosecutor has confirmed that police are searching properties in Duesseldorf and other locations for documents and evidence relating to the crash, Reuters news agency reports. Christoph Kumpa said it would take a while to assess the findings.
Andreas Lubitz lived with his parents in the town of Montabaur but also kept a flat in Duesseldorf.
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The US FBI has offered to help the French authorities with the air crash investigation. "We stand ready to fulfil any requests for information," FBI spokesman Paul Bresson told the Associated Press.
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James Longman
BBC News
tweets: "A map circling #GermanWingsCrash exclusion zone from helicopter high over the #Alps @BBCNews"
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Matthias Gebauer
Der Spiegel magazine
tweets: "Looking for a motive for the deadly descent: Police begin search warrant at #4U9525 co-driver Andreas L. Montabaur"
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Our correspondent Jenny Hill has been speaking to residents near the family home of Andreas Lubitz in Montabaur, shown below.
She reports: "'It's just horrible,' one neighbour told me as she held her little boy. She didn't know the family well at all. But she worries now that Montabaur will always be remembered for this.
"A little while ago two people strode up to the front door escorted by police officers. A woman, maybe in her 30s, carrying a notebook and a slightly older man were let into the house and the heavy brown door slammed shut again."
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Families of some of the victims have now arrived in Seyne-les-Alpes, near the crash site. The local bishop is shown here preparing to greet and support those relatives.
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BBC correspondent Jenny Hill is in Montabaur, where Germanwings co-pilot Andreas Lubitz is thought to have lived.
She says: "It's an unremarkable house in a small quiet housing estate. But this is where Andreas Lubitz is believed to have lived with his parents - and the house with grey walls and dormer windows is now the focus of the world's attention as investigators try to establish why he appears to have deliberately crashed the plane."
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She said today's news was "an additional strain on the families and in this hour full of suffering, these days full of suffering, our thoughts are especially with them".
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Mrs Merkel said that Germany would do all it could to support the investigation into the crash.
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German Chancellor Angela Merkel said she has been deeply shocked by the latest developments. "It is difficult to measure the suffering that this catastrophe has brought to so many families," she tells a news conference in Berlin. "Today, we now have received news that this tragedy has been given a new, immeasurably incomprehensible dimension."
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How are cockpit doors locked? BBC Magazine takes a look at how aircraft security systems work here.
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A 30-minute recording obtained from the cockpit voice recorder has provided the clearest indication yet as to what might have happened on board. Take a look at our analysis of the flight's final half hour.
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A coach transporting relatives of some of the victims has arrived in Seyne-les-Alpes, near the crash site in the French Alps.
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Lufthansa CEO Carsten Spohr has described the entry mechanism of the cockpit door.
"There is another code you can enter at the door which will lead to a bell ringing and if nobody reacts, the door will open automatically and allow entry," he said.
But he added that this can be blocked from the cockpit by pushing a switch that will lock the door for five minutes.
"Either the captain didn't enter the code correctly - which we find unlikely because everybody involved knows this code by heart - or the colleague had entered the code and the co-pilot prevented the door from opening by pushing the lock switch," said Mr Spohr.
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Low-cost airline Norwegian Air Shuttle will now require two people to be in the cockpit at all times for safety reasons. "When one person leaves the cockpit, two people will now have to be there," its director of operations is quoted as saying by AFP news agency.
French prosecutors say the pilot of the Germanwings plane that crashed was locked out of the cockpit by his co-pilot, who then "voluntarily" sent the plane into a descent.
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A BBC producer says she has seen police officers entering the house. It is in a well-manicured, quiet cul-de-sac on the outskirts of Montabaur, she adds.
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German police have been keeping the media away from the door of Andreas Lubitz's home in the town of Montabaur, about 60km (37 miles) north-west of Frankfurt. Neighbours have refused to comment.
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Earlier, Germany's Interior Minister Thomas de Maiziere told journalists that Andreas Lubitz was not known to the country's police or intelligence agencies. Neither, he added, had the co-pilot been flagged in any Lufthansa security checks.
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Lufthansa CEO Carsten Spohr told a news conference in Cologne that Andreas Lubitz began his training in 2008. "After his training there was an 11-month waiting period - this is not unusual with us - and since 2013 he was working as co-pilot on the Airbus A320."
He added: "I want to mention that there was a longer break in his training six years ago and after his suitability was assessed again he resumed his training. Subsequently, he passed all medical checks as well as all aviation training. He was 100% fit to fly without any restrictions or conditions."
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Members of the LSC Westerwald flying club earlier said Mr Lubitz had gained his glider pilot's licence as a teenager and that he had shown no signs of depression when he renewed that licence in the autumn.
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The chairman of Mr Lubitz's flying club, Klaus Radke, told AP that he rejected the French prosecutor's assessment that he had deliberately crashed the plane into the Alps. "I don't see how anyone can draw such conclusions before the investigation is completed."
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Members of Andreas Lubitz's flying club in Montabaur, Germany, say the 28 year old appeared to be content with his job. "He was happy he had the job with Germanwings and he was doing well," longtime LSC Westerwald member Peter Ruecker told the Associated Press. "He was very happy, he gave off a good feeling."
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Mr Spohr says: "No matter your safety regulations, no matter how high you set the bar, and we have incredibly high standards, there is no way to rule out such an event."
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Lufthansa CEO Carsten Spohr tells a news conference in Cologne: "We choose our staff very, very carefully."
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Mr Spohr says there is no indication about what the co-pilot's motive was.
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Lufthansa CEO Carsten Spohr says: "In our worst nightmares we couldn't have imagined that such a tragedy could happen within our company."
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Germany's Transport Minister, Alexander Dobrindt, said the French authorities' assessment that the plane's descent into the Alps was initiated deliberately was "according to our experts, plausible". He added: "We are in very close contact with Lufthansa regarding the conclusions that need to be drawn from this tragedy. One can only say that what the French prosecution described in great detail today is indeed more than harrowing."
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Lufthansa chairman and CEO Carsten Spohr tells a news conference in Cologne that staff at the airline, and those at its subsidiary Germanwings, are "absolutely speechless" and "deeply shocked" at the news of the co-pilot's actions. "I would not have been able to imagine that the situation could get even worse," he adds.
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Just to recap on what we have learnt in the last few hours. Officials in France say the co-pilot of the Germanwings Airbus A320 - which crashed into the French Alps on Tuesday, killing all 150 people on board - appears to have deliberately brought the plane down. Marseille prosecutor Brice Robin said Andreas Lubitz, 28, "voluntarily" sent the plane into a descent, while the pilot was locked out of the cockpit. But he said it did not appear to be a terrorist act.
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German Transport Minister Alexander Dobrindt says the emerging details of what happened on flight 4U 9525 reveal a situation that is "truly more than shocking".
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Prime Minister Mariano Rajoy of Spain says via Twitter that he is "deeply shaken by the latest information given by the investigators". He added: "Once again, our heartfelt affection to the families". There were 50 Spaniards on board the flight from Barcelona to Duesseldorf.
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The captain of Airbus A320, who has not been identified, had more than 10 years of experience with German flag carrier Lufthansa and its subsidiaries, including Germanwings. He had more than 6,000 hours of flight time, most of them on Airbus planes.
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The AFP news agency reports that the co-pilot, Andreas Lubitz, was registered as a member of a private flying club in Germany, LSC Westerwald, and was a runner who took part in local races, according to public records. This is a screengrab of what is believed to be his Facebook profile.
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On its Twitter feed, Germanwings says: "We are shaken by the upsetting statements of the French authorities. Our thoughts and prayers continue to be with the families and friends of the victims." The airline will hold a news conference at 14:30 German time (13:30 GMT).
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David Gleave, an air accident investigator whose speciality is pilot suicides, says the aviation industry needs to start looking at the psychological profiles of pilots. "We may find that this pilot had various stresses that may have led to some form of relatively irrational behaviour. It could be a vast amount of debt taken out for pilot training, it could be family matters, it could be religious matters. There's all sorts of areas that the psychological specialist will start to look at during the investigation but I certainly think that it's going to be reviewed fairly quickly."
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French prosecutor Brice Robin said the passengers on flight 4U 9525 only realised "at the last moment" they were about to crash, because "we only hear the screams on the last moments of the recording".
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Richard Westcott
BBC Transport Correspondent
This kind of event is rare but it has happened before, although the reasons vary widely. After 9/11 they made cockpits impregnable. It keeps the terrorists out, but in the end it also allows someone to keep their colleagues out too. Airlines have to make a call. Which is the bigger threat - terrorism or suicide?
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Richard Westcott
BBC Transport Correspondent
The focus now moves from the mechanics to the man flying the plane. An accident expert has told me the investigators will pore over the co-pilot's background and that of his family too. Did he owe money? Was there a grudge? They'll look at his religion, whether he was in trouble with the law, whether he had a stable love life.
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Sean Maffett, an aviation analyst, says a change in staffing levels might help prevent such incidents in future. "One possible answer to this is to insist all flights have three pilots on board, so there are always two of them on the flight deck. But airlines are run by accountants - that's horrendously much more expensive. It's a problem that is not going to be resolved straight away."
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German police have been deployed outside what is thought to be Andreas Lubitz's home in Montabaur, in west Germany.
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German Interior Minister Thomas de Maiziere says there are no indications that the co-pilot, Andreas Lubitz, had "any kind of terrorist background", the Reuters news agency reports.
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Aviation analyst Sean Maffett says there is no requirement for psychiatric tests on pilots. "There have been calls for them in the past, and the calls will be renewed for that to happen," he tells the BBC. "I know that in the pilot fraternity there is considerable worry that that is going to happen, because it'll do nothing for most people but make their lives much more difficult."
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According to the audio recording from the flight recorder, the captain is heard asking the co-pilot to take over controls of the plane. Then there is the sound of a chair being pushed back and a door being closed. Moments later the captain is heard knocking on the door and asking to be let in, but receives no answer from the co-pilot.
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Germanwings flight 4U 9525 crashed on Tuesday into the French Alps en route from Barcelona to Duesseldorf. The German authorities are now taking charge of the investigation into co-pilot Andreas Lubitz, according to French prosecutors.
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Mr Robin said pounding on the cockpit door could be heard during the final minutes of the flight of the Germanwings airliner, and that alarms sounded. He also said the co-pilot's breathing was normal throughout the final minutes.
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During the news conference, Mr Robin refused to give details on Andreas Lubitz's religion, saying: "I don't think it's necessarily what we should be looking for."
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Information taken from the black box cockpit voice recorder indicated that the co-pilot Andreas Lubitz did not say a word once the captain left the cockpit, Mr Robin said. "It was absolute silence in the cockpit."
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According to Lufthansa, which owns Germanwings, the co-pilot joined the airline in September 2013, immediately after training, and had flown 630 hours.
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Brice Robin said the responses of the co-pilot, named as Andreas Lubitz, 28, were initially courteous but became "curt" when the captain began to give a mid-flight briefing, about the planned landing of the Germanwings flight. The plane's eventual crash in the French alps killed 150 people.
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Mr Robin also said the co-pilot "pushed the button to trigger the aircraft to lose altitude. He operated this button for a reason we don't know yet, but it appears that the reason was to destroy this plane".
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Brice Robin, the state prosecutor in Marseille, told a news conference the co-pilot was alone at the controls when the plane came down. Analysis of the flight recorder showed the pilot had left the cockpit and was then calling on the co-pilot to let him back in, but there was no reply, he added. The co-pilot was heard breathing - showing that he was still alive at the time of the impact. "The most plausible interpretation is that the co-pilot through a voluntary act had refused to open the cabin door to let the captain in," Mr Robin said.
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Welcome to the BBC's live coverage of reaction to the announcement by prosecutors in the French city of Marseille that all the evidence from the cockpit voice recorder from the crashed Germanwings airliner suggests the co-pilot was responsible for the disaster, purposely steering the plane into the Alps.