According to the New York Times, an investigator said evidence from a cockpit voice recorder indicated one pilot left the cockpit before the plane’s descent and was unable to get back in.
A senior military official described a “very smooth, very cool” conversation between the pilots during the early part of the flight from Barcelona to Düsseldorf. Then the audio indicated that one of the pilots left the cockpit and could not re-enter.
“The guy outside is knocking lightly on the door and there is no answer,” the investigator said. “And then he hits the door stronger and no answer. There is never an answer.”
He said, “You can hear he is trying to smash the door down.”
“We don’t know yet the reason why one of the guys went out,” said the official, who requested anonymity because the investigation is continuing. “But what is sure is that at the very end of the flight, the other pilot is alone and does not open the door.”