PARIS—Passengers with moments to live screamed in terror and the pilot frantically pounded on the locked cockpit door as a 27-year-old German co-pilot deliberately and wordlessly smashed an Airbus carrying 150 people into an Alpine mountainside.

The account Thursday of the final moments of Germanwings Flight 9525 prompted some airlines to immediately impose stricter cockpit rules — and raised haunting questions about the motive of the co-pilot, whose breathing never wavered as he destroyed the plane and the lives of those aboard.

“We have no idea of the reason,” Marseille Prosecutor Brice Robin said, revealing the chilling conclusions investigators reached after reconstructing the final minutes of the flight from the plane’s black box voice recorder. Co-pilot Andreas Lubitz’s intention was “to destroy this plane.”

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Co-pilot loved flying, had no terrorist links

French, German and U.S. officials said there was no indication of terrorism. The prosecutor did not elaborate on why investigators do not suspect a political motive; instead they’re focusing on the co-pilot’s “personal, family and professional environment” to try to determine why he did it.

German Chancellor Angela Merkel, whose nation lost 75 people on the flight, said the conclusions brought the tragedy to a “new, simply incomprehensible dimension.” Devastated families of victims visited the crash scene Thursday, looking across a windy mountain meadow toward where their loved ones died.

The Airbus A320 was flying from Barcelona to Duesseldorf on Tuesday when it lost radio contact with air traffic controllers and began plunging from its cruising altitude of 38,000 feet, before slamming into the mountainside eight minutes later.

The final moments of Germanwings 4U 9525

The prosecutor laid out in horrifying detail the final sounds heard in the cockpit extracted from the mangled voice recorder.

Lubitz, courteous in the first part of the trip, became “curt” when the captain began the mid-flight briefing on the planned landing, Robin said.

The pilot, who has not been identified, left the cockpit for an apparent bathroom break, and Lubitz took control of the jet.

He suddenly started a manual descent, and the pilot started knocking on the door.

There was no response. “It was absolute silence in the cockpit,” the prosecutor said — except for the steady breathing he said indicated Lubitz was not panicked, and acted in a calm, deliberate manner.

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Just before the plane hit the mountain, passengers’ cries of terror could be heard.

“The victims realized just at the last moment,” Robin said. “We can hear them screaming.”

Their families “are having a hard time believing it,” he said, after briefing some of them in Marseille.

Many victims’ relatives visited an Alpine clearing Thursday where French authorities set up a viewing tent for family members to look toward the site of the crash, so steep and treacherous that it can only be reached by a long journey on foot or rappelling from a helicopter.

At the crash site, helicopters shuttled back and forth Thursday as investigators continue retrieving remains and pieces of the plane, shattered from the high-speed impact of the crash.

The principal of Joseph Koenig High School in Haltern, Germany, which lost 16 students and two teachers in the crash, said the state governor called him to tell him about the probe’s conclusion.

“It is much, much worse than we had thought,” principal Ulrich Wessel said.