Investigators have retrieved cockpit voice recordings from one of the black boxes of the German Airbus plane that crashed into the Alps, killing everyone onboard, officials said on Wednesday.

The New York Times reported that evidence from the recordings showed that one of the pilots left the cockpit and could not get back in before the flight went down.

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.

“You can hear he is trying to smash the door down,” the investigator added.

French president Francois Hollande promised that authorities would not rest until the causes of the crash were known, France’s BEA air incident investigation bureau said it was still far too early to draw meaningful conclusions on why the plane, operated by the Germanwings budget arm of Lufthansa, went down.

We have just been able to extract a useable audio data file.

- Remi Jouty, Director, BEA

“We have not yet been able to study and to establish an exact timing for all the sounds and words heard on this file,” he said.

He expected initial analysis in “a matter of days” but that rough read-out could be subject to errors and that more work would be needed for a full interpretation.

Although he said “words” had been heard on the tape, Jouty would not confirm whether that meant the Airbus A320’s pilots were conscious and he gave no details of the recordings.

The New York Times quoted a senior military official involved in the investigation as saying the cockpit audio showed “very smooth, very cool” conversation between the pilots in the early part of the flight. The audio then indicated one of the pilots left the cockpit.

We don’t know yet the reason why one of the guys went out. But what is sure is that at the very end of the flight, the other pilot is alone and does not open the door.

Interior Minister Bernard Cazeneuve earlier said a terror attack was not the main hypothesis being worked on. Jouty said, however, that no theory could be excluded at this point.

While stressing it was too early to form a clear picture, he ruled out a mid-air explosion having taken place and said the crash scenario did not appear to be linked to depressurisation.

Jouty said the second black box had not yet been found but could not confirm Hollande’s remark that its casing had.

It’s the first major air passenger disaster on French soil since the 2000 Concorde accident just outside Paris. Germanwings said 72 Germans were killed in Tuesday’s crash while the Spanish officials said 51 Spaniards were among the victims.

As well as Germans and Spaniards, victims included three Americans, a Moroccan and citizens of Britain, Argentina, Australia, Belgium, Colombia, Denmark, Israel, Japan, Mexico, Iran and the Netherlands, officials said.