The co-pilot of doomed Germanwings Flight 9525 deliberately acted to “destroy the plane,” as he was alone in the cockpit and manually guided the aircraft on its disastrous descent into a mountainside in the French Alps, an investigator said Thursday.

The co-pilot, Andreas Lubitz, said “not one word” as the flight's pilot desperately tried to reenter the locked cockpit during the Airbus A320's eight-minute descent into death, which claimed the lives of 150 passengers and crew near Seyne, France, on Tuesday.

Lubitz, a 28-year-old German living in Montabaur, Germany, locked the cockpit after the captain got up to use the bathroom, Marseille prosecutor Brice Robin said during a press conference. One of the plane's flight-data recorders revealed that Lubitz did not panic, his breathing remaining steady, as he piloted the passengers to their deaths. The descent from cruising altitude into the mountainside, which covered roughly 32,000 feet, required Lubitz to maneuver a lever "multiple times," Robin said, and could not have been done automatically.

"We conclude for all circumstances that it was deliberate," Robin said.

There was no response from the cockpit despite several calls from air-traffic controllers on the ground. "There was no answer whatsoever; no answer to their many calls," Robin said.

The captain, who has not been named, was heard on a cockpit recording banging on the door repeatedly and yelling for it to be opened.

Screams from passengers were only heard at “the very last moments before impact,” Robin said, indicating they didn’t know their fate until just before the end.

“Normally people committing suicide do it on their own," the prosecutor said. "When responsible for 150 people behind you, I don’t consider that a suicide.”

Lubitz was not on any terrorist watch list, Robin said. He had only 100 hours of flight experience on an Airbus A320 but was "fully qualified" to pilot the aircraft, Robin said. The captain, meanwhile, was much more experienced, having had some 10,000 flight hours, Robin said.

Alarms could be heard on the recording, Robin said, just before the plane plowed into the mountain at 435 mph.

"Death was instantaneous," Robin said.

Prior to the crash, Lubitz gave "curt" answers to his captain during a discussion pertaining to the flight checklist, the prosecutor said.

The Germanwings plane was traveling from Barcelona to Dusseldorf and was less than an hour into the flight when it crashed. Robin said families of the victims were "in shock" when he told them the crash was the result of a deliberate action by the co-pilot.

"They found it difficult to believe," Robin said. "I tried to give them answers ... I answered the best I could."

Germanwings and Lufthansa confirmed the prosecutor's account during a later press conference, calling it a "tragic individual event."

Lubitz passed all tests and "was 100% fit to fly without restrictions" after passing all flight and medical examinations.