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.
Meanwhile, the head of Lufthansa, the German carrier that owns Germanwings, said the pilot had undergone intensive training and "was 100% fit to fly without any caveats".
"We have no findings at all about what motivated the pilot to do this terrible deed," Carsten Spohr said.
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".
Mr Robin 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.
Mr Lubitz, 28, was alive until the final impact, the prosecutor said.
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 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.
"This action on the altitude controls can only be deliberate."
He added: "The most plausible interpretation is that the co-pilot through a voluntary act had refused to open the cabin door to let the captain in. He pushed the button to trigger the aircraft to lose altitude. He operated this button for a reason we don't know yet, but it appears that the reason was to destroy this plane."
He said the co-pilot was "not known by us" to have any links to extremism or terrorism.
But he said German authorities were expected to give further information on his background and private life later.
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, which owns Germanwings, 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.