BERLIN--The Germanwings co-pilot who is believed to have crashed an airliner into a French mountainside had been undergoing psychotherapy because of suicidal thoughts over an extended period of time before he obtained his pilot license, the German prosecutors in charge of the case said Monday.

The statement confirms comments from a person close to the investigation who had said Andreas Lubitz had been treated for depression, a fact he concealed from his employer.

"A few years ago, the co-pilot had been in psycho-therapeutic treatment with noted suicide risk over a long period of time before he gained his pilot licence," Düsseldorf prosecutor Ralf Herrenbrück said in a statement. "In the ensuing period and up until now, further visits to the doctors that resulted in excuses from work took place without suicide risk or aggression against third parties."

The prosecutors said their investigation and interviews with witnesses had so far uncovered no sign that Mr. Lubitz was planning to deliberately crash an airplane. Neither could they establish a motive for such an act, they said.

The medical documents seized in the course of the investigation showed no evidence that Mr. Lubitz was suffering from any physical ailments.

French prosecutors have said Mr. Lubitz, who was 27 years old, appeared to have locked the captain of Germanwings Flight 9525 out of the cockpit, programmed the A320's descent and slammed it into an Alpine ridge at 400 miles an hour on Tuesday, killing all 150 people aboard.

Write to Andrea Thomas at andrea.thomas@wsj.com

Access Investor Kit for Deutsche Lufthansa AG

Visit http://www.companyspotlight.com/partner?cp_code=P479&isin=DE0008232125

Access Investor Kit for Deutsche Lufthansa AG

Visit http://www.companyspotlight.com/partner?cp_code=P479&isin=US2515613048

Subscribe to WSJ: http://online.wsj.com?mod=djnwires