The co-pilot of the crashed Germanwings jet barricaded himself in the cockpit and "intentionally" sent the plane full-speed into a mountain in the French Alps, a prosecutor has said.
Co-pilot Andreas Lubitz's "intention (was) to destroy this plane", Marseille prosecutor Brice Robin said, laying out the conclusions reached by French aviation investigators after listening to the last minutes of Tuesday's Flight 9525.
Mr Robin said just before the plane hit the mountain, the sounds of passengers screaming could be heard on black box audio. "I think the victims realised just at the last moment," he said.
The Airbus A320 was flying from Barcelona to Duesseldorf when it began to descend from cruising altitude of 38,000 feet after losing radio contact with air traffic controllers. All 150 people on board died when the plane slammed into the mountain.
Mr Robin said the pilot left the cockpit, presumably to go to the lavatory, and then was unable to regain access. In the meantime, Lubitz, a 27-year-old German, manually set the plane on the descent that drove it into the mountain.
He said the commander of the plane knocked several times "without response". He said the door could only be blocked manually.
"The most plausible, the most probably, is that the co-pilot voluntarily refused to open the door of the cockpit for the captain and pressed the button for the descent," Mr Robin said.
He said the co-pilot's responses, initially courteous in the first part of the trip, 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 Mr Robin said the co-pilot said nothing from the moment the commanding pilot left. "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 the whole time, Mr 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. German authorities are taking charge of the investigation into the co-pilot.
The A320 is designed with safeguards to allow emergency entry if a pilot inside is unresponsive, but the crew's override code does not work - and goes into a lockdown - if the person inside the cockpit specifically denies entry, according to an Airbus training video and a pilot who has six years of experience with the jets.
Airlines in Europe are not required to have two people in the cockpit at all times, unlike the standard US operating procedure after the 9/11 attacks which was changed to require a flight attendant to take the spot of a briefly departing pilot.
In the German town of Montabaur, acquaintances said Lubitz appeared normal and happy when they saw him last autumn 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."
Lufthansa CEO Carsten Spohr said the airline was already "appalled" by what happened before today's details were revealed.
"I could not have imagined that becoming even worse," he said in Cologne. "We choose our cockpit staff very, very carefully."
The families of victims were briefed about the conclusions just ahead of the announcement.
"The victims deserve explanations from the prosecutor," Mr Robin said. "They are having a hard time believing it."
He said the second black box still had not been found but remains of victims and DNA identification have begun.
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, Mr Ruecker said. He described Lubitz as a "rather quiet" but friendly young man.
His recently deleted Facebook page appeared to show a smiling man in a dark brown jacket posing in front of the Golden Gate Bridge in California. The page was wiped at some time in the past two days.
Lufthansa said Lubitz joined Germanwings in September 2013, directly out of flight school, 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.
German chancellor Angela Merkel said news that the co-pilot is believed to have intentionally crashed the plane gives the tragedy a "new, simply incomprehensible dimension".
She said that "something like this goes beyond anything we can imagine" and underlined a pledge that German authorities will do "everything imaginable to support the investigations".
The principal of Joseph Koenig High School in Haltern, which lost 16 students and two teachers in the crash, said the state governor called him with news that the cause "was without a doubt suicide".
Ulrich Wessel said: "I gave this information to my colleagues immediately, and they were just as stunned as I was.
"I told them it is much, much worse than we had thought. It doesn't make the number of dead any worse, but if it had been a technical defect then measures could have been taken so that it would never happen again."
Lufthansa's chief executive, Carsten Spohr, said "no system in the world can rule out such an isolated event".
He added: "I have worked at Lufthansa as an engineer, I have worked as a pilot at Lufthansa, I have carried responsibility as a manager at Lufthansa for many, many years.
"Always, wherever I was, whoever my boss was, the rule was always safety is number one. And that this has happened to us - I can only say we are sorry."
Many families have visited an Alpine clearing near the scene of the crash.
French authorities set up a viewing tent in the hamlet of Le Vernet for family members to look toward the site of the crash.
The area itself is so steep and treacherous it can only be reached by a long journey on foot or rappelling from a helicopter.
The Lubitz family is in France but being kept separate from the other families, Mr Robin said.
Helicopters shuttled back and forth form the crash site, as investigators continue retrieving remains and pieces of the plane, shattered from the high-speed impact of the crash.