Co-pilot Andreas Lubitz appears to have intentionally crashed a Germanwings plane into the French Alps, according to a Marseille prosecutor.
All 150 people on board the Airbus A320 were killed in Tuesday's crash.
Andreas Lubitz, 28, had been working for the Germanwings budget carrier, a subsidiary of Lufthansa, since 2013.
He lived with his parents at their home in the western town of Montabaur - where investigators are now working.
Mr Lubitz first took to the skies as a teenager at a glider club in his home town. He learned to fly a two-seat glider when he was about 14 or 15 years old, according to the club's chairman Klaus Radke.
In 2008 he was accepted as a Lufthansa trainee, after obtaining his glider pilot's license, and enrolled at the company's training school in Bremen.
Six years later he joined subsidiary airline Germanwings and began working as a co-pilot.
The German national had just 630 hours of flying time, compared with the 6000 hours of the flight captain.
Lufthansa chief executive Carsten Spohr said its air crew were picked carefully and subjected to psychological vetting.
"He was 100 percent fit to fly without any restrictions or conditions," said Mr Sphor.
He said Mr Lubitz's training had been interrupted for several months six years ago, but was resumed after "the suitability of the candidate was re-established".
Acquaintances in Montabaur told Reuters they were stunned, describing him as an affable young man who gave no indication he was harbouring any harmful intent.
"I'm just speechless. I don't have any explanation for this. Knowing Andreas, this is just inconceivable for me," said Peter Ruecker, a long-time member of the local flight club where Mr Lubitz received his flying licence years ago.
"He was a lot of fun, even though he was perhaps sometimes a bit quiet. He was just another boy like so many others here."
He appeared to have led an active lifestyle, running a half-marathon in a good time and showing an interest in pop music and night-clubs, according to his Facebook page.
The recently deleted page bearing his name showed him as a smiling man in a brown jacket posing in front of the Golden Gate bridge in California.
French prosecutor Brice Robin said the 28-year-old locked the doors of the cockpit after the captain went to the bathroom and sent the plane into descent,
Mr Robin said there were no grounds to suspect that Mr Lubitz had carried out a terrorist attack. He refused to discuss his religious background.
"Suicide" was also the wrong word to describe actions which killed so many other people, he said. "I don't necessarily call it suicide when you have responsibility for 100 or so lives."
- Reuters / BBC / AFP