SEYNE-LES-ALPES, France — Lufthansa’s chief executive said Wednesday it will take “a long, long time” to understand what led to a deadly crash in the French Alps last week — but refused to say what the airline knew about the mental health of the co-pilot suspected of deliberately destroying the plane.

Lufthansa CEO Carsten Spohr and the head of its low-cost airline Germanwings, Thomas Winkelmann, visited the crash area Wednesday amid mounting questions about how much the airlines knew about co-pilot Andreas Lubitz’s psychological state and why they haven’t released more information about it.

The two men laid flowers and then stood silently facing a stone monument to the plane’s 150 victims. The monument looks toward the mountains where the Germanwings A320 crashed and shattered into thousands of pieces March 24. It bears a memorial message in German, Spanish, French and English.

Spohr said the airline is “learning more every day” about what might have led to the crash but “it will take a long, long time to understand how this could happen.”

He then deflected questions from reporters at the site in Seyne-les-Alpes and drove away.

Based on audio from the plane’s voice data recorder, investigators believe Lubitz intentionally crashed the plane and are trying to figure out why.

Lufthansa acknowledged Tuesday that it knew six years ago that Lubitz had suffered from an episode of “severe depression” before he finished his flight training at the German airline, but said he had passed all his medical checks since then.

The airline did not mention the severe depression episode when questions were raised last week about Lubitz’s medical history.

German prosecutors say Lubitz’s medical records from before he received his pilot’s license referred to “suicidal tendencies,” but visits to doctors since then showed no record of any suicidal tendencies or aggression against others.

The revelations intensify questions about how much Lufthansa and its insurers will pay in damages for the passengers who died — and about how thoroughly the aviation industry and government regulators screen pilots for psychological problems.