One of the pilots of the crashed Germanwings jet can be heard in a 'black box' recording leaving the cockpit, then banging on the door with increasing urgency in an unsuccessful attempt to get back in, a US newspaper report has suggested.
The mangled device recovered from the wreckage of has yielded sounds and voices, but not the "slightest explanation" of why the plane crashed on a mountain in the southern French Alps, killing all 150 on board, investigators have said officially.
The New York Times quoted an unidentified investigator as saying: "The guy outside is knocking lightly on the door and there is no answer.
"And then he hits the door stronger and no answer. There is never an answer."
Eventually, the investigator says, "you can hear he is trying to smash the door down."
The investigator, whom the newspaper said could not be identified because the crash probe was continuing, said officials did not know why the pilot left.
He did not speculate on why the other pilot did not open the door or make contact with ground control before Tuesday's crash.
Lufthansa has not identified the pilots or given details of ages and nationality, but it said the co-pilot joined Greenwings in September 2013, directly after training, and had flown 630 hours.
The captain had more than 6,000 hours of flying time and had been a Germanwings pilot since May last year, having previously flown for Lufthansa and Condor, Lufthansa said.
The first half of Germanwings Flight 9525 was chillingly normal.
It took off from Barcelona en route to Duesseldorf, climbing up over the Mediterranean and turning over France.
The last communication was a routine request to continue on its route.
Minutes later, at 10.30am, the Airbus A320 inexplicably began to descend.
Within 10 minutes it had plunged from its cruising altitude of 38,000 feet to just over 6,000 feet and slammed into a remote mountainside in Seyne-les-Alpes.
Since the September 11 2001 terrorist attacks, airlines in the US do not leave one pilot alone in the cockpit.
The standard operating procedure is that if one of the pilots leaves, for example to use the bathroom, a flight attendant takes their spot in the cockpit.
It was not immediately clear if European airlines have adopted the same practice.
The names of the pilots have not been released.
French officials gave no details from the recording, insisting the cause of the crash remained a mystery.
They said the descent was gradual enough to suggest the plane was under the control of its navigators.
"At this point, there is no explanation," Remi Jouty, head of France's accident investigation bureau BEA, said.
"One doesn't imagine that the pilot consciously sends his plane into a mountain."
Mr Jouty said "sounds and voices" were registered on the digital audio file recovered from the first black box. But he did not divulge the contents, insisting days or weeks will be needed to decipher them.
"There's work of understanding voices, sounds, alarms, attribution of different voices," he said.
Confusion surrounded the fate of the second black box.
French president Francois Hollande said the casing of the flight data recorder had been found in the scattered debris, but was missing the memory card that captures 25 hours' worth of information on the position and condition of almost every major part in a plane. Mr Jouty refused to confirm the discovery.
French officials said terrorism appeared unlikely and Germany's top security official said there was no evidence of foul play.
As authorities struggled to unravel the puzzle, Mr Hollande, German chancellor Angela Merkel and Spanish prime minister Mariano Rajoy converged on the remote accident site to pay their respects to the dead - mostly German and Spanish citizens among at least 17 nationalities.
"This is a true tragedy, and the visit here has shown us that," Mrs Merkel said after she and Mr Hollande overflew the desolate craggy mountainside.
Helicopters ferried in rescue workers and other personnel throughout the day. More than 600 rescue and security workers and aviation investigators were on site, French officials said.
Germanwings CEO Thomas Winkelmann said the budget airline, a subsidiary of Lufthansa, was contacting victims' families.
Two babies, two opera singers and 16 German school pupils and their teachers returning from an exchange programme in Spain were among those who lost their lives.
The principal of Joseph Koenig High School, Ulrich Wessel, called the loss a "tragedy that renders one speechless".
In Spain, flags flew at half-mast on government buildings and a minute of silence was held in government offices across the country and parliament cancelled its session.
Barcelona's Liceu opera house held two minutes of silence at noon to honour the two German opera singers, Oleg Bryjak and Maria Radner, who were returning home after a weekend performance at the theatre.
Germanwings cancelled several flights yesterday because some crews declared themselves unfit to fly after losing colleagues.