SEYNE-LES-ALPES, France (AP) — Authorities at the site of the Germanwings plane crash in the French Alps say they've finished collecting human remains. 

They say there are no longer any visible remains at the site. One police official says there are only "belongings and pieces of metal" remaining on the ground.

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.

Meanwhile, the chief executive of Lufthansa says it will take a "long, long time" to understand what led to last week's deadly crash.

But Carsten Spohr is refusing to say what the airline knew about the mental health of the co-pilot who's suspected of deliberately destroying the plane.

Spohr and the head of Lufthansa's low-cost airline Germanwings visited the crash area today. They laid flowers and then stood silently facing a stone monument to the 150 people who died. The monument looks toward the mountains where the plane crashed. It bears a memorial message in German, Spanish, French and English.

Spohr says the airline is "learning more every day" about what might have led to the crash.

The airline acknowledged yesterday that it knew six years ago that Andreas Lubitz had suffered from an episode of "severe depression" before he finished his flight training. But it said he had passed all of his medical checks since then.