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 emerging of the German co-pilot's past - although his apparent motives for causing the crash remain a mystery.
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Mr Lubitz, 27, 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 for several months six years ago, but did not say why.
The training was resumed after "the suitability of the candidate was re-established", he said.
German Chancellor Angela Merkel told reporters that the co-pilot's apparent actions had given the tragedy a "new, simply incomprehensible dimension".
Police have been searching the co-pilot's home in Montabaur, near Frankfurt, as well as a flat he kept in Duesseldorf.
The Airbus 320 from Barcelona to Duesseldorf hit a mountain, killing all 144 passengers and six crew, after an eight-minute descent.
Who was Andreas Lubitz?
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"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.
(BBC)
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