Almost 10 minutes before panicked screams echoed through the aisles as Flight 9525 plunged toward the French Alps, Andreas Lubitz — a 27-year-old electronic-music buff who had clawed his way up from flight steward to co-pilot — was silent and alone in the cockpit.

The Airbus A320 had cruised to 38,000 feet amid cheerful banter between Lubitz and his far more experienced pilot. But as the pilot shifted his attention to paperwork for landing the short-haul Germanwings flight from Barcelona to Düsseldorf, Lubitz grew less animated, offering brief responses. At 10:31 a.m. Tuesday, the pilot — named in the German media only as “Patrick S” — apparently heeded nature’s call, rustling out of his seat and exiting the cockpit, never to get back in.

Moments later, Lubitz, according to a French review of recovered flight data, took the A320, which was carrying 150 passengers and crew members, off autopilot and began a controlled descent that initially would not have seemed unusual to those aboard. Then came the knocking — increasingly frantic — by the pilot as he sought to reenter the locked and reinforced cockpit door. In the final moments, the sounds of terrified passengers filled the plane even as Lubitz — audibly breathing as a bleeping alarm warned of imminent collision — kept quiet through the end.

On Thursday, the tragedy turned from air disaster to criminal investigation as authorities in multiple nations scoured for clues to what could have compelled a man to hurl a packed commercial airliner into a mountain. Germanwings parent company Lufthansa on Thursday expressed stunned shock, describing Lubitz as “100 percent fit to fly.”

As officials carted out boxes of belongings, including a laptop, from his family’s home in a ­middle-class neighborhood of this southwestern German town, questions centered on several months in 2009 when Lubitz took a leave from his pilot training. Those who knew him, however, could not reconcile the reserved young pilot and avid runner who lived with his parents with the accounts of French prosecutor Brice Robin, who said Thursday that Lubitz’s actions appeared to be a deliberate attempt “to destroy the plane.”

The dramatic revelations from the black-box recordings, meanwhile, seemed to challenge a fundamental faith of flying — the sanity of the people at the controls. In the wake of Sept. 11, 2001, cockpit doors have been re­designed for strength to keep people out, but leaving planes vulnerable to a danger that instead lies within.

The possible scenario of a lone pilot willfully crashing a jetliner additionally highlighted the differences between airlines in the United States — which do not allow one person to remain alone in a cockpit — and European airlines, which do.

On Thursday, some of the biggest German airline companies — among them Lufthansa and Air Berlin — agreed to new regulations that would prohibit pilots from being left alone in the cockpit. The new regulation was set to be discussed Friday with Germany’s Federal Aviation Office.

Saying that the French and the German investigations were suggesting that the airplane’s ­co-pilot deliberately crashed the plane, German Chancellor Angela Merkel told reporters in Berlin, “It goes beyond the imagination.”

Lufthansa said it could not disclose any details about the hiatus Lubitz took in 2009, citing German privacy laws protecting medical confidentiality. But if the company had been informed of any serious mental health issue, safety regulations should not have allowed him to continue training or remain in the air.

In Montabaur, a town of 13,000, police stood outside Lubitz’s house — a two-story home with eyelid windows — as a German prosecutor and other officials searched the inside. Later in the evening, they carted off large blue plastic bags filled with evidence without speaking to reporters. One official familiar with the investigation said authorities had not yet found anything like a suicide note, but the official cautioned that the search had just begun. German and French officials said there were no indications Lubitz belonged to a terrorist organization.

At the same time, details of Lubitz’s life were gradually coming to light. A longtime aficionado who dreamed of flying planes as a youth, he belonged to his local flight club and, after a stint as a cabin attendant, landed a coveted spot in Lufthansa’s pilot training program in 2008. He did his training, which can take 33 months, at company facilities in Bremen, Germany, and Phoenix.

“Andreas became a member of the association and wanted his dream of flying to be realized. He began in the gliding school and made it to become a pilot,” read a statement on the Web site of his club, Luftsportclub Westerwald, that was posted before authorities outlined the contents of the data recordings.

Lubitz was relatively new to the post of co-pilot at Germanwings, the budget arm of Lufthansa. He had been in the job for 18 months, logging 630 hours of flight experience — enough, authorities said, to safely manage the plane on his own. Lufthansa chief executive Carsten Spohr said Lubitz passed the company’s stringent physical and psychological tests.

“We at Lufthansa are speechless,” Spohr told reporters.

Neighbors here in Lubitz’s home town described him as a reserved but “sweet” man who enjoyed long runs through the local woods and had a winning smile. One neighbor who would not give his name said he had known Lubitz since he was a child and did not believe that the 27-year-old had been on a suicide mission. “I have my theories about what happened, but someone who would do something like that would not run through the woods to keep fit,” the neighbor said.

But Robin, the prosecutor, offered a chilling account of Flight 9525, saying the plane made a steep but steady descent that did not appear to startle most passengers until it was clear it was on a collision course with the snowy peaks in southern France.

“The screams are not heard until the very last moments,” Robin said.

The jetliner made a late takeoff just after 10 a.m. local time from Barcelona and the last contact with air-traffic controllers came at 10:30 a.m., Robin said. “Direct IRMAR merci 18G,” one of the pilots says, referring to a passage point south of Barcelonnette, in the French Alps, and giving the call sign of the plane in an indication that everything is going well.

At 10.31 a.m., after the pilot is heard to exit and while the plane is at its cruising altitude of 38,000 feet, data recordings show that the autopilot is turned off and the co-pilot activates the flight monitoring system to start the descent of the plane. An analysis of transponder data by air tracker Flightradar24 showed that the plane’s autopilot had been manually reset from 38,000 to below 100 feet.

“This action can only be done deliberately,” Robin said.

When the pilot tries to reenter, the recordings show, he becomes increasingly frantic, banging on the door when the co-pilot does not let him in. Although a code exists for entering the cockpit, door-locking overrides can be applied from inside of the cockpit — a post-9/11 modification.

Despite repeated attempts by air traffic control to contact Lubitz, Robin said there were no more replies from the plane’s cockpit, which remained eerily quiet as panic ensued.

The French prosecutor’s statement raised parallels with the rare cases of apparently intentional crashes of passenger planes.

In 1999, an Egypt Air plane went into a steep plunge after taking off from New York bound for Cairo, crashing into the Atlantic and killing all 217 people aboard. Investigators concluded that a mechanical malfunction was highly unlikely.

In 1994, the pilot on a Royal Air Maroc flight appeared to intentionally slam the plane into a Moroccan mountainside. All 44 people aboard were killed.

Faiola reported from Antwerp, Belgium. Souad Mekhennet in Antwerp and Virgile Demoustier in Paris contributed to this report.

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