The co-pilot of the Germanwings flight that crashed in the French Alps, named as Andreas Lubitz, appeared to want to "destroy theplane", 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.