One of the pilots got out of the cockpit of the Germanwings Airbus A320 and could not get back in, the New York Times quotes a "senior military official" as saying.

The plane with 150 people on board was flying from Barcelona in Spain to D?sseldorf in Germany when it went down and plowed into the mountains near the French town of Barcelonette, 100 km north of Nice.

Comments from the official came after the French Bureau of Investigations and Analysis retrieved the information stored in the cockpit voice recorder.

The agency's head Remi Jouty then said obtaining "usable" information from the recordings could take days.

But the New York Times quotes the unnamed official as saying that, following a "very smooth, very cool" conversation in the early part of the flight, a pilot who left the cockpit failed to reenter.

"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."

The official is described as an "investigator".

Authorities are trying to establish what exactly led to the crash, which seems to have happened in good weather, and with the pilot sending no distress signal from the cockpit after the plain started inexplicably to descend.

The last message sent out was at 10:30 CET, and ten minutes later the aircraft had sunk at a speed said to be two to three times the average for landing approach.

Officials are yet to investigate, and most sources are basing their conclusions on preliminary information.

Wednesday, Tuesday and Friday have been declared days of mourning by the EU Commission.