Family and relatives of the victims of the Germanwings plane crash in the Alps are taken on bus to the Prat airport from a hotel in Castelldefels in Barcelona, Spain, to take a Lufthansa flight to visit the crash site in Seyne les Alps in France, March 26, 2015. [Photo/IC]

"Flying was his life," said the friend, who agreed to speak to Reuters about Lubitz's mental state on condition of anonymity. "He always used to be a quiet companion, but in the last year that got worse."

Randy Knipping, a Toronto-based specialist in aviation medicine, said it was extremely difficult to conduct screening for psychiatric problems among pilots, in part because they canput their careers in jeopardy by admitting to such problems.

"If somebody wants to withhold or conceal, there's no reasonable test," he said, adding that unless a family member comes forward to flag a mental problem, then problem cases can "slip through".

In Montabaur, a small town of 12,000 near the Rhine River,friends who knew Lubitz at the local flying club were stunned and saddened. Some pleaded with reporters who descended on the town not to rush to judge Lubitz until all the facts are known.

"I'm just speechless. I don't have any explanation for this. Knowing Andreas, this is just inconceivable for me," Peter Ruecker, a long-time member of the flight club, told Reuters.