Pressure grew Wednesday for German airline Lufthansa to explain how Andreas Lubitz — who had exhibited suicidal tendencies and temporarily left a pilot training program due to depression — could have been left at the helm of an ill-fated Germanwings flight.

Lufthansa chief executive Carsten Spohr declined to comment on when the airline learned of mental health problems affecting the 27-year-old co-pilot, who is believed to have intentionally crashed the plane into a mountainside in France last week, killing himself and all 149 others on board.

“We are learning more every day, but it will take a long, long time to understand how this could happen,” Spohr said during a visit to a memorial near the crash site. Lufthansa is Germanwings’ parent company.

“We’re very, very sorry that such an accident could have happened at Lufthansa, where we put so much focus on safety. We are sorry for the losses that occurred and there are just no words to express this,” Spohr said.

Lufthansa had said its flight training school knew of Lubitz’s problems with depression. The company on Tuesday produced a 2009 email in which Lubitz explained he was ready to resume flight training after a break.

In it, Lubitz informed the school of a “previous episode of severe depression,” sparking questions about why his history did not raise red flags at the airline.

Lufthansa provided the email along with other documents to prosecutors and said it was cooperating fully with the investigation.

Prosecutors said Lubitz had been in treatment with psychiatrists and neurologists, and that a doctor had noted before he received his pilot’s license that he had suicidal tendencies.

Earlier, Spohr said Lubitz had passed all his medical tests and that he “was fit for flying without any restrictions.” Medical records outlining the suicidal tendencies were protected under German confidentiality laws, and it is unclear if Germanwings was aware of that section of Lubitz’s file.

He also passed medical and psychological evaluations to obtain a student pilot certificate from the U.S. Federal Aviation Authority in 2010.

In Berlin, a spokeswoman for the German Transport Ministry said it would not consider changes to the country’s pilot selection standards until prosecutors have finished their investigation.

A spokesman for the French recovery team gathering evidence from the crash site said that there were no longer any human remains visible. A group of four German investigators specializing in digital and laser tracing arrived Wednesday to collect evidence.

In the town of Haltern in the west of Germany, families and friends gathered late Wednesday for a religious service in memory of the 16 pupils and two teachers from a local school killed in the crash.

The students from the town’s Joseph Koenig high school had been returning from a trip to Spain.

Germany’s national memorial service for the dead is set for April 17 in the Catholic cathedral of Cologne with Chancellor Angela Merkel attending.