MONTABAUR, Germany—The man suspected of flying himself and 149 people to their deaths on Tuesday was a quiet, 27-year-old German who had a passion for gliders and competitive running, and an unexplained gap in his flight training, according to his employer and people who knew him.

Andreas Lubitz, the co-pilot who authorities say appeared to fly Germanwings Flight 9525 intentionally into the French Alps, didn’t fit the suicidal profile suggested by French authorities’ early investigation, these people said.

“He wasn’t an extroverted guy,” said Peter Rücker, a 64-year-old retiree who handles maintenance at the LSC Westerwald flight club, where Mr. Lubitz enrolled as a teenager before beginning his commercial-pilot training. But “he was very responsible and fit in well (at the club),” he said.

Earlier on Thursday, the French prosecutor in charge of the crash probe said voice-recorder evidence indicated that Mr. Lubitz locked himself inside the cockpit, preventing the more experienced pilot from re-entering after he had left. Then Mr. Lubitz appeared to take steps to intentionally crash the jet, initiating the plane’s fatal descent while ignoring the pilot’s pleas to let him back in, the prosecutor said.

Carsten Spohr, chief executive of Deutsche Lufthansa AG , the budget carrier’s parent, said he had no indications as to why Mr. Lubitz would have intentionally crashed the jet.