SEYNE-LES-ALPES, France — The last communication from a doomed German jetliner was routine. The mangled black box has yielded sounds and voices, the lead investigator said Wednesday, but so far not the “slightest explanation” why the plane plunged into an Alpine mountainside, killing all 150 on board.

Midway through a flight from Barcelona to Duesseldorf on Tuesday, Germanwings Flight 9525 was chilling in its normalcy. The last communication was a routine request to continue on its route, said Remi Jouty, the head of the accident investigation bureau.

In this photo provided by the French Interior Ministry rescue workers investigate on the crash site near Seyne-les-Alpes, Wednesday. The Associated Press

Then minutes later, at 10:30 a.m., the Airbus A320 inexplicably began to descend. Within 10 minutes it had plunged almost 32,000 feet from its cruising altitude of 38,000 feet, slamming into the remote mountainside at an elevation just above 6,000 feet, Jouty said.

With no distress call or other indication of anything out of the ordinary, French investigators struggled Wednesday to solve the mystery. While alarming, the descent was still gradual enough to suggest the plane was under the control of its navigators.

“The descent is compatible with a plane controlled by pilots,” Jouty said. “It is also compatible with a plane controlled by automatic pilot.”

“At this point, there is no explanation,” he added. “One doesn’t imagine that the pilot consciously sends his plane into a mountain.”

The New York Times is reporting that the voice recorder indicates one of the pilots was locked out of the cockpit before the crash.

The newspaper is citing an investigator it doesn’t identify as saying that the audio shows that after an ordinary start to the flight, one of the pilots left the cockpit and could not get back in.

The investigator told the newspaper that the pilot began knocking quietly on the door, then became more insistent, saying that eventually: “You can hear he is trying to smash the door down.”

The investigator does not speculate as to why the other pilot didn’t open the door or make contact with ground control before the plane crashed.

Jouty said “sounds and voices” were registered on the digital audio file in the first black box, which was retrieved six hours after the crash and delivered Wednesday to investigators. He did not divulge the contents, insisting time is needed to decipher them.

Confusion surrounded the fate of the second black box. French President Francois Hollande said the casing of the flight data recorder had been found in the scattered debris, but was missing the memory card that captures 25 hours’ worth of information on the position and condition of almost every major part in a plane. However, Jouty refused to confirm the discovery

French officials said terrorism appeared unlikely and Germany’s top security official said there was no evidence of a crime.

Hollande, German Chancellor Angela Merkel and Spanish Prime Minister Mariano Rajoy converged on the remote accident site to pay their respects to the dead – mostly German and Spanish citizens among at least 17 nationalities.

“This is a true tragedy, and the visit here has shown us that,” Merkel said after she and Hollande overflew the desolate craggy mountainside.

Jouty said the voice recorder had yielded sounds and voices, though he refused to specify whether they belonged to the pilots or divulge what was said. He said deciphering the material is “ongoing work” that could take days or weeks.

“There’s work of understanding voices, sounds, alarms, attribution of different voices,” he said.

Lufthansa CEO Carsten Spohr, himself a pilot, said he found the crash of a plane flown by two experienced captains while at cruising altitude “inexplicable.” Cruise is considered the safest part of a flight, with only about 10 percent of crashes occurring at that altitude.

“We still cannot understand what happened there yesterday,” he said. “Lufthansa has never in its history lost an aircraft in cruise flight and we cannot understand how an airplane that was in perfect technical condition, with two experienced and trained Lufthansa pilots, was involved in such a terrible accident.”

The four possible causes of any crash are human error, mechanical problems, weather, criminal activity or any combination. Investigators use the cockpit voice and flight data recorders to focus their work, said Alan E. Diehl, a former air safety investigator with the National Transportation Safety Board.

“Both will point you in directions of what is critical,” Diehl says. “Based on what you learn from the recorders, you might focus on key pieces of wreckage.”

“You’re usually dealing with a jigsaw puzzle with many of the pieces missing,” he said. “You start eliminating things that didn’t happen.”

The flight data recorder is important because of the critical information it provides on the plane’s altitude, speed, location and condition. But experts said the analysis can begin with the voice recordings investigators already have.

“You have a moment in time and you can start linking what you hear to what you have seen happening on the radar trace,” said Joris Melkert, an aerospace engineer at Delft University of Technology in the Netherlands.

“Are these pilots fully conscious? Are they doing things? Are they just chatting? Do they recognize that something is wrong? … Do you hear the sound of alarm bells? Are there spoken messages from the aircraft that are warning something has gone wrong?”

Helicopters ferried in rescue workers and other personnel throughout the day. More than 600 rescue and security workers and aviation investigators were on site, French officials said.

Germanwings CEO Thomas Winkelmann said the airline was contacting victims’ families. He said the 144 passengers and six crew members included 72 Germans, 35 Spaniards, three Americans and two people each from Australia, Argentina, Iran, Venezuela, and one person each from Britain, the Netherlands, Colombia, Mexico, Japan, Denmark, Belgium and Israel.

The three Americans included a mother and daughter, the U.S. State Department said. Some of the victims may have had dual nationalities; Spain’s government said 51 citizens had died in the crash.

Two babies, two opera singers and 16 German high school students and their teachers returning from an exchange program in Spain were among those who lost their lives.

The principal of Joseph Koenig High School, Ulrich Wessel, called the loss a “tragedy that renders one speechless.”

In Spain, flags flew at half-staff on government buildings and a minute of silence was held in government offices across the country. Parliament canceled its Wednesday session.

Barcelona’s Liceu opera house held two minutes of silence at noon to honor the two German opera singers, Oleg Bryjak and Maria Radner, who were returning home after a weekend performance at the theater.

Germanwings canceled several flights Wednesday because some crews declared themselves unfit to fly after losing colleagues.

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