One of the two pilots of the Germanwings plane that crashed in the French Alps was locked out of the cockpit, according to reports.
Early findings from the cockpit voice recorder suggest the pilot made desperate efforts to get back in, sources close to the investigation say.
The Airbus 320 from Barcelona to Duesseldorf hit a mountain on Tuesday after a rapid eight-minute descent.
Relatives of the 150 passengers and crew who died are to visit the area.
Lufthansa, which owns Germanwings, is operating two special flights on Thursday – one from Barcelona and one from Duesseldorf – to Marseille, and both groups will travel on by road.
Germanwings chief Thomas Winkelmann said 72 passengers on flight 4U 9525 were German citizens, including 16 pupils returning from an exchange trip.
Spain’s government said 51 of the dead were Spanish.
Other victims were from Australia, Argentina, Britain, Iran, Venezuela, the US, the Netherlands, Colombia, Mexico, Japan, Denmark and Israel.
On Wednesday, French officials said usable data had been extracted from the cockpit voice recorder but that it was too early to draw any conclusions.
Remi Jouty, director of the French aviation investigative agency, said he hoped investigators would have the “first rough ideas in a matter of days” but the full analysis could take weeks or even months.
An unnamed investigator as saying that one of the pilots – it is not clear if it is the captain or the first officer – left the cockpit and had been unable to get back in.
“The guy outside is knocking lightly on the door, and there is no answer,” the investigator said, describing audio from the recorder.
“And then he hits the door stronger, and no answer. There is never an answer. You can hear he is trying to smash the door down.”
A source close to the investigation told a similar story to the AFP news agency.
An alarm indicating proximity to the ground can be heard before the moment of impact, the source adds.
Lufthansa has not named the pilots but it said the co-pilot joined Germanwings in September 2013, directly after training, and had flown 630 hours.
It said the captain had more than 6,000 hours of flying experience and had been with Germanwings since May 2014, having flown previously for Lufthansa and Condor.
Mr Jouty said the second “black box” – the flight data recorder – had not been found and he could not confirm an earlier statement by President Francois Hollande that its casing had been recovered.
Mr Jouty said controllers observed the plane beginning to descend and tried to contact the pilots but without success.
He ruled out an explosion, saying: “The plane was flying right to the end.”
Mr Jouty said: “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.”
Families and friends of the victims are expected to arrive at the crash site at Meolans-Revels later on Thursday.
Separately, a bus carrying 14 relatives of Spanish victims left Barcelona on Wednesday for the crash area, because they did not want to fly.
In France, special teams have been prepared to assist the families during their visit.