Relatives and friends of the 150 passengers and crew on Germanwings Flight 4U 9525 are due to go to the crash site high in the French Alps.

Lufthansa will operate two special flights - one from Barcelona and one from Duesseldorf to Marseille, and both groups will travel on by road.

Reports say one of the two pilots on the doomed flight had left the cockpit and had been unable to get back in just before the crash on Tuesday.

There were no survivors, officials say.

They say the Airbus 320 from Barcelona to Duesseldorf hit a mountain after a rapid eight-minute descent.

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.

Germanwings is a low-cost airline owned by Germany's main carrier Lufthansa.

Cockpit mystery

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, French officials said usable data had been extracted from the cockpit voice recorder of the Germanwings plane.

Remi Jouty, the 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 Jouty said this was not the case.

'Flying to the end'

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.

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."

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."

'Darkest hour'

On Wednesday, President Francois Hollande, German Chancellor Angela Merkel and Spanish Prime Minister Mariano Rajoy gave a press conference after visiting the crash site.

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 Merkel said they would do everything they could to find out the cause of the crash.

In a press briefing later, Carsten Spohr, CEO of Lufthansa, which owns Germanwings, said this was "the darkest hour of 60 years of Lufthansa".

He added: "We cannot understand how an airplane in perfect technical condition with two such trained pilots was involved in such a terrible accident."