SEYNE-LES-ALPES, France - The first half of Germanwings Flight 9525 was chilling in its normalcy. It took off from Barcelona en route to Dusseldorf, climbing up over the Mediterranean and turning over France. The last communication was a routine request to continue on its route.
Minutes later, at 10:30 a.m. Tuesday, the Airbus A320 inexplicably began to descend. Within 10 minutes it had plunged from its cruising altitude of 38,000 feet to just over 6,000 feet and slammed into a remote mountainside.
To find out why, investigators have been analyzing the mangled black box that contains an audio recording from the cockpit. Remi Jouty, the head of France's accident investigation bureau BEA, said Wednesday that it has yielded sounds and voices, but so far not the "slightest explanation" of why the plane crashed, killing all 150 on board.
A newspaper report, however, suggests the audio contains intriguing information at the least: One of the pilots is heard leaving the cockpit, then banging on the door with increasing urgency in an unsuccessful attempt to get back in.
"The guy outside is knocking lightly on the door and there is no answer," the New York Times quotes an unidentified investigator as saying. "And then he hits the door stronger and no answer. There is never an answer."
Eventually, the newspaper quotes the investigator as saying: "You can hear he is trying to smash the door down."
The investigator, whom the newspaper said could not be identified because the investigation is continuing, said officials don't know why the pilot left.
He also does not speculate on why the other pilot didn't open the door or make contact with ground control before the crash.
Since the 9/11 terrorist attacks, airlines in the United States don't leave one pilot alone in the cockpit.
The standard operating procedure is that if one of the pilots leaves - for example to use the bathroom - a flight attendant takes his or her spot in the cockpit. It was not immediately clear whether European airlines have adopted the same practice.
The names of the pilots have not been released.
French officials gave no details from the recording on Wednesday, insisting the cause of the crash remained a mystery.
They said the descent was gradual enough to suggest the plane was under the control of its navigators.
"At this point, there is no explanation," Jouty said. "One doesn't imagine that the pilot consciously sends his plane into a mountain."
Jouty said "sounds and voices" were registered on the digital audio file recovered from the first black box. But he did not divulge the contents, insisting days or weeks will be needed to decipher them.
"There's work of understanding voices, sounds, alarms, attribution of different voices," the BEA chief said.