The boss of the airline involved in the French Alps disaster said today he was "stunned" that the plane might have been deliberately crashed by its co-pilot.
Lufthansa chief executive Carsten Spohr said the crash, which killed 150 people, including co-pilot Andreas Lubitz, was "beyond our worst nightmare".
He was speaking in Cologne after French prosecutors released evidence from the recovered cockpit voice recorder of the plane operated by Lufthansa's budget airline, Germanwings.
It showed that Mr Lubitz appeared to have sent the jet crashing into the mountains, killing everyone on board, including at least three Britons.
German Mr Lubitz put the plane into a descent after locking the captain out of the cockpit, Marseille prosecutor Brice Robin said.
Evidence from the black box flight recorder suggests he then refused to open the cockpit door to the captain, who can be heard pounding on the door in a desperate attempt to break in.
Mr Spohr said: "Not in our worst nightmare would we imagine this happening. This is by far the most terrible event for our company."
He said Mr Lubitz was fit to fly and his previous performance has "without any criticism".
The evidence appears to show that, although the locked-out captain punched the emergency number into the cockpit door to gain entry, the co-pilot inside then relocked the door.
Mr Spohr said that, irrespective of all the sophisticated safety devices, "you can never exclude such an individual event", adding "no system in the world could manage to do that".
He added: "We can only speculate what might have been the motivation of the co-pilot. In a company that prides itself on its safety record, this is a shock. We select cockpit personnel carefully."
Earlier, in Marseille, Mr Robin said: "The most probable interpretation is that the co-pilot refused to open the cockpit door to the pilot and actioned the button which started the descent procedure.
"We can only deduce that it destroyed this plane."
The recording also suggests the passengers were unaware of what was happening until the final moments, when their screams can be heard.
Mr Robin said the cockpit voice recorder gave information from the first 30 minutes of the flight.
For the first 20 minutes the two pilots talked in a normal fashion and were as courteous as two pilots would be.
He said the co-pilot's responses, initially courteous, became "curt" when the captain began the mid-flight briefing on the planned landing.
The captain is then heard asking the co-pilot to take over and the sound of a chair being pushed back and a door being closed is heard.
Moments later the captain can be heard knocking on the door and asking to be let in but there is no answer from the co-pilot.
Asked about Mr Lubitz's ethnicity, Mr Robin said: "He was a German national and I don't know his ethnic background.
"He is not listed as a terrorist, if that is what you are insinuating."
Pressed on the co-pilot's religion, he said: "I don't think this is where this lies. I don't think we will get any answers there."
He said German authorities were taking charge of the investigation of Mr Lubitz.
Mr Robin said black box recordings showed that Mr Lubitz "was breathing normally, it wasn't the breathing of someone who was struggling".
Speaking about whether the passengers realised what was happening, Mr Robin said: "I think the victims only realised at the last moment because on the recording we only hear the screams on the last moments of the recording."
He added: "I believe that we owe the families the transparency of what the investigation is pointing to and what is going on, we owe it to them to tell them what happened.
"The families have been informed of everything I just told you."
It was assumed that the captain had gone to the toilet, leaving the co-pilot in charge of the plane, the prosecutor said.
Mr Robin went on: "The co-pilot uses the flight monitoring system to start the descent of the plane. This can only be done voluntarily, not automatically.
"We hear several cries from the captain asking to get in. Through the intercom system he identifies himself - but there is no answer. He knocks on the door and asks for it to be opened - but there is no answer."
Mr Robin said that after entry to the cockpit was denied, the sound of breathing from inside the cockpit was heard and this sound carried on until the moment of impact.
"The co-pilot was still alive at this point," Mr Robin said.
He said there was no distress signal, no Mayday and no answer despite numerous calls to the plane from ground controllers.
The cockpit voice recorder also shows that there were alarm signals going off, indicating the proximity of the ground.
Noises of someone trying to break down the cockpit door are heard, then finally the sound of an impact. Mr Robin said the plane may have glided before the moment of impact.
Acquaintances in Mr Lubitz's home town of Montabaur in western Germany said he was happy and had showed no signs of depression when they last saw him.
One described him as quiet but friendly.
Jim McAuslan, general secretary of British airline pilots' association Balpa, said: "All pilots will be shocked at what has happened and will be thinking of the captain who was trying to gain access to the flight deck to save the passengers.
"When this detailed investigation is concluded and all the facts are available, pilots want airlines, safety regulators and manufacturers to work with us to ensure lessons are learned from this tragedy and steps are taken so that it does not happen again."
Airbus said: "Our thoughts are with everyone affected by this appalling tragedy, above all, of course, the victims and their family members and friends.
"Our condolences also go to our colleagues at Germanwings and Lufthansa. At this very difficult time, we will continue to provide all possible help and support."
The German chancellor Angela Merkel said: "I have visited the site of the accident and the forces at work there, we thank them for their very difficult work and promise that our government will do everything to help to investigate, without any omission, all the circumstances of this crash.
"Today's news is an additional strain on the families in this hour of suffering. In these days of suffering our thoughts are especially with them again.
"I repeat my promise that I made yesterday on my visit to the crash site to all citizens, especially the families and relatives from all countries who have lost dear ones in this catastrophe.
"The federal government and the German authorities will do everything thinkable in order to support the investigation.
"We owe this to all the people who lost their lives as well as their relatives who are suffering so terribly now."
Matthias Gebauer, chief correspondent for the online edition of Der Spiegel newspaper, tweeted: "Schoolmates of co-pilot who crashed #4U9525 tell German reporters he took 6-months break from flight training in 2009 due to burnout-syndrome."
Downing Street said David Cameron was being kept up to date with the latest developments and an Air Accidents Investigation Branch (AAIB) official was heading to France.
A Number 10 spokeswoman said: "Clearly we are following the situation closely, the Prime Minister is being kept updated.
"The AAIB are sending someone to join the French-led investigation, given that there are victims from the UK in the crash."
The UK's Civil Aviation Authority (CAA) said: "Following the details that have emerged regarding the tragic Germanwings incident, we are co-ordinating closely with colleagues at the European Aviation Safety Agency (EASA) and have contacted all UK operators to require them to review all relevant procedures.
"All UK airline pilots undergo extensive and regular medical assessments to determine their fitness to hold a licence. As part of this, aeromedical examiners are required to assess a commercial pilot's mental health at each medical examination which, for an airline pilot flying with at least one other pilot, is undertaken annually."
The CAA spokesman added: "These detailed medical assessments are in line with international aviation standards.
"We will continue to monitor the situation as the investigation develops and our thoughts remain with the friends and relatives of all those affected by this tragic incident."
A GERMANWINGS jet carrying 150 people from Barcelona to Duesseldorf slammed into a remote section of the French Alps, sounding like an avalanche as it scattered pulverised debris across a rocky mountain and down its steep ravines.