Co-pilot Andreas Lubitz - pictured here on his social media profile - is not known to have any links to terrorism
The co-pilot of the Germanwings flight that crashed in the French Alps, named as Andreas Lubitz, appeared to want to "destroy the plane", officials said.
Marseille prosecutor Brice Robin, citing information from the "black box" voice recorder, said the co-pilot was alone in the cockpit.
He intentionally started a descent while the pilot was locked out.
Mr Robin said there was "absolute silence in the cockpit" as the pilot fought to re-enter it.
He said air traffic controllers made repeated attempts to contact the aircraft, but to no avail. Passengers could be heard screaming just before the crash, he added.
Details are also emerging of the co-pilot's past - although his apparent motives for causing the crash remain a mystery.
Mr Lubitz, 28, had undergone intensive training and "was 100% fit to fly without any caveats", according to Carsten Spohr, the head of Lufthansa, the German carrier that owns Germanwings.
Mr Spohr said Mr Lubitz's training had been interrupted briefly six years ago but was resumed after "the suitability of the candidate was re-established".
A view of the cockpit of the Germanwings aircraft, photographed a few days before the crash
The Airbus 320 from Barcelona to Duesseldorf hit a mountain, killing all 144 passengers and six crew, after an eight-minute descent.
"We hear the pilot ask the co-pilot to take control of the plane and we hear at the same time the sound of a seat moving backwards and the sound of a door closing," Mr Robin told reporters.
He said the pilot, named in the German media as Patrick S, had probably gone to the toilet.
"At that moment, the co-pilot is controlling the plane by himself. While he is alone, the co-pilot presses the buttons of the flight monitoring system to put into action the descent of the aeroplane.
"He operated this button for a reason we don't know yet, but it appears that the reason was to destroy this plane."
Mr Lubitz was alive until the final impact, the prosecutor said.
Mr Robin said "the most plausible interpretation" was that the co-pilot had deliberately barred the pilot from re-entering the cockpit.
He added that the co-pilot was "not known by us" to have any links to extremism or terrorism.
The focus now moves from the mechanics to the man flying the plane. An accident expert has told me the investigators will pore over the co-pilot's background and that of his family too.
Did he owe money? Was there a grudge? They'll look at his religion, whether he was in trouble with the law, whether he had a stable love life. This kind of event is rare but it has happened before, although the reasons vary widely.
After 9/11, they made cockpits impregnable. It keeps the terrorists out, but in the end it also allows someone to keep their colleagues out too. Airlines have to make a call. Which is the bigger threat - terrorism or suicide?
Passengers were not aware of the impending crash "until the very last moment" when screams could be heard, Mr Robin said, adding that they died instantly.
Meanwhile, relatives and friends of the victims are due to visit the area of the crash.
Lufthansa has arranged two special flights for families and friends on Thursday - one from Barcelona and one from Duesseldorf - to Marseille, and both groups will travel on by road. Separately, some relatives who did not want to fly are travelling by bus from Barcelona.
The second "black box" - that records flight data - has still not been found.
Source: Aviation Safety Network