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 site.
Lufthansa will operate 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 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 of the Germanwings plane.
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.
He said he hoped investigators would have the “first rough ideas in a matter of days” but that the full analysis could take weeks or even months.
But 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 adds, describing audio from the recorder.
A source close to the investigation told a similar story to the AFP news agency.
There had been earlier reports that the second black box – the flight data recorder – had been found. But Mr Jouty said this was not the case.
‘Flying to the end’
Mr Jouty said the plane’s last communication was a routine one with air traffic control.
The plane confirmed instructions to continue on its planned flight path but then began its descent a minute later.
Mr Jouty said controllers observed the plane 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.”
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.
On Wednesday, President Francois Hollande, German Chancellor Angela Merkel and Spanish Prime Minister Mariano Rajoy gave a press conference after visiting the crash site.
Mr Hollande told his counterparts: “The French people are here shoulder to shoulder with you during this ordeal. Everything will be done to find, identify and hand back to the families the bodies of their loved ones.”
Both he and Mrs Merkel said they would do everything they could to find out the cause of the crash.
Germanwings is a low-cost airline owned by Germany’s main carrier Lufthansa.
In a press briefing, Carsten Spohr, CEO of Lufthansa, said this was “the darkest hour of 60 years of Lufthansa”.
He added: “We cannot understand how a plane in perfect technical condition with two such trained pilots was involved in such a terrible accident.”
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