The 28-year-old co-pilot "deliberately" initiated the plane's descent, Marseille prosecutor Brice Robin told a press conference in which he revealed the details of recordings made by the Airbus' cockpit flight recorder in the final minutes before the crash that killed all 150 aboard.
"The co-pilot is alone at the controls," Robin said. "He voluntarily refused to open the door of the cockpit to the pilot and voluntarily began the descent of the plane."
"At this moment, there is no indication that this is an act of terrorism," Robin said, adding that the co-pilot, identified as Andreas Lubitz, had no known terrorist connection. "He was not known to us."
Despite the horrifying scenario of a rogue co-pilot sending the Germanwings crew and passengers to their deaths, Robin said that the passengers on board probably were unaware "until the end".
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"The screams aren't heard until the end," he said.
All six crew and 144 passengers died "instantly," he added.
The shocking new information was released as families and friends of victims began arriving in France to travel to the remote mountainous crash site area, where locals have opened their doors in a show of solidarity with the grieving relatives.
Two planes arrived in southern France today from Barcelona and Duesseldorf with families and friends.
Tents were set up for them near the crash site area to give DNA samples to start the process of identifying the bodies of loved ones, at least 51 of whom were Spaniards and at least 72 Germans.
A mountain guide who got near the crash site said he was unable to make out recognisable body parts.