LAUSANNE, Switzerland - The copilot who is suspected of deliberately crashing a plane loaded with 150 people into the French Alps may have had a brief, untreated bout of depression while undergoing pilot training, according to German news accounts.

Lufthansa, the operator of the Germanwings airline, said that the copilot, Andreas Guenter Lubitz, had taken a break during his training but had been thoroughly tested and cleared upon his return, completed his training, and was considered "100 percent fit and ready to fly."

But the mother of a former classmate told the German newspaper Frankfurter Allgemeine that Lubitz had confessed to her daughter a few years ago that his timeout during training was because of "a burnout, a depression."

Still, the copilot was well thought of, and people who knew him were at a loss to explain what would have caused him to decide to kill all those aboard in a rare deliberate crash of a passenger aircraft.

"People who commit suicide usually do so alone. When you do it with 150 people behind you, it's not suicide," said the French prosecutor leading the investigation, Brice Robin. "That is why I am not using this word. I don't call it a suicide."

Robin said that Lubitz had no known connection to terrorism and that there is no known motive for his apparent actions.

Lubitz was said to have 620 hours of flight time, only a tenth of the flight experience of the pilot. Still, while young, he was well thought of. A September 2013 article by Aviation Business Gazette noted that "the Federal Aviation Administration (FAA) is recognizing Andreas Guenter Lubitz with inclusion in the prestigious FAA Airmen Certification Database. The database . . . names Lubitz and other certified pilots who have met or exceeded the high educational, licensing and medical standards established by the FAA."

After news of the crash first broke, there were reports in the German media that many in Lubitz's village of Montabaur were deeply saddened, noting that flying had been his lifelong dream. After Thursday's news broke, however, that sympathy turned to shock at Lubitz, the son of a church organist.

Facebook pages labeling him a monster popped up within minutes of Robin's news conference and soon after topped 1,000 likes.

But L.S.C. Westerwald, the glider club where Lubitz had first learned to fly, remained loyal. Fellow club member Peter Ruecker told Austrian television that Lubitz "was a very nice young man. I have no explanation for this."

And the club chairman, Klaus Radke, urged caution. "All pilots undergo regular evaluations, their physical and mental fitness is tested. I would urge you not to draw conclusions too fast," he said. "Planes have fallen out of the sky before, for no apparent reason."