The German pilot who crashed a plane in the French Alpslast week, killing 150 people, told officials at a Lufthansa training school in 2009 that he had gone through a period of severe depression, the airline said on Tuesday.

The statement is potentially damaging for the airline and its CEO Carsten Spohr, who told reporters last week that the carrier knew of no reason why 27-year-old Andreas Lubitz might deliberately crash a plane.

The fact that Lufthansa officials were aware that Lubitz suffered from depression raises questions about its screening process for pilots as it faces the threat of legal action from relatives of the victims.

Lufthansa said Lubitz broke off his pilot training for a period of several months but then passed medical checks confirming his fitness to fly.

When Lubitz resumed training in 2009, he provided the flight school with medical documents showing that he had gone through a "previous episode of severe depression," Lufthansa said, citing emailed correspondence between Lubitz and the flight school.

Duesseldorf state prosecutors said on Monday Lubitz had been treated for suicidal tendencies before getting his pilot's licence.

They last week found torn-up sick notes showing that Lubitz was suffering from an illness that should have grounded him.

Germanwings said it had not received a sick note from Lubitz for the day of the crash. Lubitz had a valid medical certificate at the time of the crash of the Airbus A320 operated by its budget unit, Lufthansa added.

Lufthansa said it had passed the email correspondence and additional documents to the Duesseldorf prosecutors after internal investigations.

Psychiatric screening

Lufthansa was already facing unlimited liability for damages in the crash, lawyers have said, and has told its insurers to set aside $300 million to deal with claims, recovery costs and the loss of the aircraft.

For Germanwings to limit its liability, it would have to establish that it and its employees and agents were not in any way at fault or that the accident had been caused solely by the fault of a third party.

Lubitz's statement about severe depression raises more questions about what the flight school and Lufthansa did, or did not do, to make sure he was fit to fly, said New York lawyer Justin Green, who represents families in aviation disasters.

"Based on the facts as they are now known, I find it hard to accept that the airline did not have the opportunity to discover that he was a danger," Green wrote in an email. "I do not expect the airline will try to have a liability fight under the circumstances."

European pilots pass psychiatric screening ahead of starting training but annual medical checks do not include a specific in-depth psychiatric test. Pilots are expected to inform medical examiners at their annual checks of any health issues that could affect their flying ability.

Psychiatrists and pilots have said there is no fail-safe test to spot suicidal pilots, but lawyers representing some of the families of victims of the Germanwings crash called on Tuesday for more regular psychiatric testing of pilots.

French air accident investigators also said on Tuesday that their probe into the crash would look at psychological screening procedures for pilots and cockpit access, indicating the two areas where they are most likely to make safety recommendations that could affect the whole aviation industry.

"In these cases, when you're acting for families, they're keen to see that lessons are learned, that pressure is put on airline authorities to improve flight safety," Jim Morris, a partner at law firm Irwin Mitchell, told Reuters.