A young German co-pilot locked himself alone in the cockpit of a Germanwings airliner and flew it into a mountain with what appeared to be deliberate intent to crash, a French prosecutor said. Investigators and grieving relatives were left struggling to explain what motivated Andreas Lubitz, 28, to kill all 150 people on board the Airbus A320, including himself, in Wednesday's crash in the French Alps.French and German officials said there was no indication that the crash was a terrorist attack, but gave no other explanation.But Matthias Gebauer, Der Spiegel magazine's online chief correspondent, tweeted that Lubitz may have suffered from depression or burnout.Gebauer said he spoke to Lubitz's neighbours and friends, who said he was struck by depression during the several months he took out of his pilot training in 2009.Lubitz gained sole control of the aircraft after the captain left the cockpit, refused to open the door and then sent the plane into its fatal descent, Marseille prosecutor Brice Robin said.He did this "for a reason we cannot fathom right now, but which looks like intent to destroy this aircraft".He said there were no grounds to suspect that Lubitz was carrying out a terrorist attack.Robin said sound recordings from one of the doomed plane's black boxes suggested most of the passengers would not have been aware of their fate until the y end. "Only towards the end do you hear screams," he said. " Bear in mind that death would have been instantaneous. The aircraft was literally smashed to bits."The CEO of Lufthansa, parent company of Germanwings, said its air crew were picked carefully and subjected to psychological vetting.Lubitz, who joined the budget carrier in September 2013, had just 630 hours of flying time - compared with the 6000 hours of the flight captain, named in German media only as "Patrick S". ..