No distress signal was sent from the Airbus A320 and the crew failed to respond to ground control's desperate attempts to make contact.
The cockpit recording of the flight showed the pilots speaking normally in German at the start of the flight, the source said, adding it could not be determined if it was the captain or the first officer who left the cockpit.
The New York Times cited a senior military official involved in the investigation who had heard the cockpit voice recorder as saying that there was a "very smooth, very cool" conversation between the two pilots during the early part of the doomed flight.
The audio then indicated that one of the pilots left the cockpit and could not re-enter, the investigator said.
"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 official reportedly said: "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."
Before the pilot information emerged yesterday and in the absence of solid information, the thinking of aviation-safety experts and pilots had coalesced around a theory of rapid decompression that incapacitated the flight crew - perhaps because a window had blown out or the seal on a cargo hatch had failed.
Investigators will also want to know whether there was anything in the hold, either in luggage or cargo, that may have contributed to the disaster.
Timeline to disaster
Bild
published this timeline (times are local):
10:30
Germanwings flight 4U 9525, call sign "Germanwings One Eight Golf", confirmed instructions from French air traffic control.
10:31.02
Flight 4U 9525 leaves its assigned cruising altitude without approval and begins to descend. Radar observes an average descent rate of approximately 17.8m per second. Attempts by French air traffic control to contact the flight on the assigned radio frequency are not answered.
10:35.08
Attempts to contact the flight on the international distress frequency are also unsuccessful.
10:36
French air traffic control declares an international normalised emergency according to international norms. French search and rescue services are informed. Flight 4U 9525 passes through an altitude of around 7600m.
10:36.47
French air traffic control tries again to contact Flight 4U 9525 on the international distress frequency. There is no response.
10:40
Flight 4U 9525 disappears from radar. The last known altitude was about 1890m.
10:42
French air traffic control informs the search and rescue national control centre of the loss of radar contact.
10:49
Two military search and rescue helicopters head towards the location of Flight 4U 9525's final radar contact. There is no report of the aircraft's Emergency Locator Transmitter (ELT) being detected.
11:10
The wreckage of Flight 4U 9525 is identified by search and rescue helicopters.
-Telegraph Group Ltd, AFP, Independent
