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.