KUALA LUMPUR, March 26 — The pilot of Germanwings Flight 9525 that crashed in the French Alps was locked out of his cockpit and desperately trying to regain entry before the plane went down, according to a recording from the plane’s cockpit voice recorder.

According to a senior military official attached to the ongoing investigation into the crash that killed all 150 on board, the flight had been uneventful up until the point the pilot sought to re-enter the cockpit, the New York Times (NYT) reported today.

“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 unnamed official told the US daily.

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

But the revelation from the just-recovered “black box” has only added to the mystery about why the Airbus A320 crashed without ever sending a distress signal.

The plane descended rapidly from 38,000 ft and smashed into the mountainous terrain with sufficient velocity to kill all 144 of the mostly German and Spanish passengers along the with six crew members and strew the aircraft’s debris over several kilometres.

“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,” the official told the NYT.

Investigators have not found the flight data recorder in their bid to piece together information about the crash, but have ruled out a mid-air explosion or rapid cabin depressurisation.

France’s Interior Minister Bernard Cazeneuve previously said a terror attack was not the main hypothesis for the crash, but the country’s BEA director Remi Jouty said this could not be ruled out yet.

Among factors perplexing investigators are the plane’s sudden and apparently controlled descent from its cruising altitude and the clear weather when the crash occurred. Another is the complete silence in the cockpit while the descent began, up until just before the plane crashed.

“I don’t like it... To me, it seems very weird: this very long descent at normal speed without any communications, though the weather was absolutely clear,” one French official who is part of the probe told the NYT.

Carsten Spohr, the chief executive of Lufthansa that operates Germanwings, said this was the first time the airline had a plane crash while cruising.

The A320 is one of the most commonly flown commercial passenger planes and is rated positively.

“It is inexplicable this could happen to a plane free of technical problems and with an experienced, Lufthansa-trained pilot,” Spohr was quoted as saying by news agency Reuters.

Flight 9525 crashed into the French side of the Alps while flying from Barcelona, Spain to Dusseldorf in Germany on May 24, killing all 150 people on board.

It is the first major civilian air disaster in France since 2000 and the deadliest since a 1981 crash that killed 180 people.