Lufthansa's chief executive has said it will take "a long, long time" to understand what led to a deadly crash in the Alps last week.

But he 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, were visiting the crash area amid mounting questions about how much the airlines knew about co-pilot Andreas Lubitz's psychological state and why they have not released more information about it.

The two men lay 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 on March 24 and bears a memorial message in German, Spanish, French and English.

Mr 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.

After listening to the plane's voice data recorder, investigators believe Lubitz intentionally crashed the plane.

Lufthansa acknowledged yesterday that it knew Lubitz had suffered from an episode of "severe depression" before he finished his flight training at the German airline, but that he has passed all his medical checks since.

German prosecutors say Lubitz's medical records from before he received his pilot's licence 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.

At the crash site today, authorities said they have finished collecting human remains.

"(We) will continue looking for bodies, but at the crash site there are no longer any visible remains," said Lieutenant Colonel Jean-Marc Menichini.

Lieutenant Luc Poussel said all that's left are "belongings and pieces of metal".

Officials at France's national criminal laboratory near Paris say it will take a few months for the painstaking identification process to be complete and for the remains to be returned to the families.