He was 27, loved to fly and apparently raised no suspicions or showed obvious signs that he was troubled.

The co-pilot of the fatal Germanwings Flight 9525 from Barcelona, Spain, to Düsseldorf, Germany, was identified as Andreas Lubitz, who grew up in Montabaur, a Rhineland town of some 15,000 people, a neat, pretty place of timber and brick homes in western Germany.

In a tragedy full of unanswered questions from the moment he was said to have turned the Airbus A320’s nose downward over the French Alps on Tuesday morning, Mr. Lubitz has emerged as the most terrifying mystery of all.

As the world absorbed the news Thursday that he is said to have suicidally or murderously driven the jetliner into a mountainside, taking 149 people to their deaths along with him, the focus turned to what had driven him to such an act — and to whether the airline industry and regulators do enough to screen pilots for psychological problems.

There were few immediate answers or easily discernible signs of a young man in trouble, only a small unexplained gap in his training record.

Peter Rücker, a member of the flight club where Mr. Lubitz learned to fly, told Reuters television on Thursday that he knew the young man as a cheerful, careful pilot, and that he could not imagine him committing such an act.

Online, Mr. Lubitz appeared to be a keen runner, including at Lufthansa’s Frankfurt sports club, and had completed several half-marathons and other medium-distance races, including an annual New Year’s run in Montabaur in 2014.