The co-pilot of the Germanwings plane that crashed into the French Alps on Tuesday appeared to want to "destroy the plane", French 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. However, passengers could be heard screaming just before the crash, he added.
The co-pilot, now named as Andreas 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 a rapid 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.
"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."
Mr Robin said that air traffic controllers made repeated attempts to contact the aircraft, but to no avail.
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.
Meanwhile. relatives and friends of the victims are due to visit the area of the crash.