PARIS - As a frantic pilot pounded on the cockpit door and passengers screamed in panic, the Germanwings co-pilot "intentionally" sent Flight 9525 straight into the side of a mountain in the French Alps, a prosecutor said Thursday.In a news conference in Paris, Marseille prosecutor Brice Robin laid out the horrifying conclusions reached by French aviation investigators after listening to the last minutes of the Tuesday morning flight from Barcelona to Duesseldorf. The Airbus A320 began to descend from cruising altitude after losing radio contact with ground control and slammed into the remote mountain, killing all 150 people on board.It was the co-pilot's "intention to destroy this plane," Robin said.He said the pilot, who has not been identified, left the cockpit, presumably to go to the lavatory, and then was unable to regain access. In the meantime, co-pilot Andreas Lubitz, a 28-year-old German, manually and "intentionally" set the plane on the descent that drove it into the mountain.Robin said the commander of the plane knocked several times "without response." He said the door could only be blocked manually.He said the co-pilot's responses, initially courteous, became "curt" when the captain began the mid-flight briefing on the planned landing.The information was pulled from the black box cockpit voice recorder, but Robin said the co-pilot did not say a word after the commanding pilot left the cockpit."It was absolute silence in the cockpit," he said.During the final minutes of the flight's descent, pounding could be heard on the cockpit door as plane alarms sounded but the co-pilot's breathing was normal throughout the whole time, Robin said."It's obvious this co-pilot took advantage of the commander's absence. Could he have known he would leave? It is too early to say," he said.He said Lubitz had never been flagged as a terrorist and would not give details on his religion or ethnic background. He said German authorities were taking charge of the investigation into Lubitz.Robin said just before the plane hit the mountain, the sounds of passengers screaming could be heard on the audio."I think the victims realized just at the last moment, " he said.The families of victims were briefed about the shocking conclusions just ahead of the announcement."The victims deserve explanations from the prosecutor," Robin said. "(But) they have having a hard time believing it."Robin said the second black box still had not been found but remains of victims and DNA identification have begun, he said.In the German town of Montabaur, acquaintances told The Associated Press that Lubitz showed no signs of depression when they saw him last fall as he renewed his glider pilot's license."He was happy he had the job with Germanwings and he was doing well," said a member of the glider club, Peter Ruecker, who watched Lubitz learn to fly. "He gave off a good feeling."Lubitz had obtained his glider pilot's license as a teenager, and was accepted as a Lufthansa pilot trainee after finishing a tough German college preparatory school, Ruecker said. He described Lubitz as a "rather quiet" but friendly young man.Lufthansa said the co-pilot joined Germanwings in September 2013, directly after training, and had flown 630 hours.The captain had more than 6,000 hours of flying time and been a Germanwings pilot since May 2014, having previously flown for Lufthansa and Condor, Lufthansa said.Copyright 2015 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.