One of the two pilots of the Germanwings aircraft that crashed in the French Alps was locked out of the cockpit, according to unconfirmed reports.
Early findings from the cockpit voice recorder suggest the pilot made desperate efforts to get back in, sources close to the investigation reportedly said.
The Airbus 320 from Barcelona to Duesseldorf hit a mountain on Tuesday killing all 150 people on board including three Britons after a rapid eight-minute descent.
The New York Times quoted an unnamed investigator as saying that one of the pilots had left the cockpit and had been unable to get back in.
“You can hear he is trying to smash the door down,” the investigator added, describing audio from the recorder.
A source close to the investigation told a similar story to the AFP news agency.
Remi Jouty, director of the French aviation investigative agency, said there were sounds and voices on the cockpit voice recorder but that it was too early to draw any conclusions.
The last communication from the aircraft was a routine one with air traffic control.
It confirmed instructions to continue on its planned flight path but then began its descent a minute later.
Jouty said controllers observed the Airbus beginning to descend and tried to get back in contact with the pilots but without success.
He ruled out an explosion, saying: “The plane was flying right to the end.”
The BBC reported Jouty as saying: “At this stage, clearly, we are not in a position to have the slightest explanation or interpretation of the reasons that could have led this plane to descend… or the reasons why it did not respond to attempts to contact it by air traffic controllers.”
Germanwings said in a statement: “There are media reports as to which one of the pilots supposedly has left the cockpit and was not able to get back in.
“We have not received any information from the authorities leading the investigation and therefore can neither confirm nor deny the reports of the New York Times.
“We are working on obtaining more information but will not participate in any kind of speculation.
“The investigation on what caused the accident falls to the responsible authorities.”
A businessman from Wolverhampton, a hospitality student from Hull and a seven month old baby from Manchester were named as the British passengers on board the doomed flight.