The co-pilot who crashed Flight 9525 into a French mountainside last week had informed the German carrier Lufthansa in 2009 about a "previous episode of severe depression," the airline said on Tuesday, raising fresh questions about the series of decisions that allowed Andreas Lubitz to stay in the skies.

The admission that the company knew at least some of the history of Lubitz's mental illness came after the company's chief executive Carsten Spohr said publicly last week that Lufthansa — parent of the budget airline Germanwings for which Lubitz worked — had no previous knowledge of his medical history.

In a statement on Tuesday, however, the carrier said it wanted to issue a "swift and seamless clarification." In 2009, Lubitz had taken several months off during his training to become a pilot. When he resumed the program, Lufthansa said, he provided them "medical documents" that noted his bout of severe depression.

The company said it had forwarded those documents to prosecutors who are now handling the crash as a homicide case.

Under European aviation law, pilots with active and untreated cases of depression are prevented from flying. But if deemed medically cured, there may have been no legal impediment for Lubitz to continue his training and obtain his license, experts say.

However, pilots who have attempted "a single self-destructive act" — such as suicide — are legally barred from commercial flying. Also, pilots who are taking psychotropic medications — such as popular anti-depressants — as part of their therapy, for instance, have some limitations, including a stipulation that they not be alone in the cockpit.

German prosecutors said Monday that Lubitz had suffered from "suicidal tendencies" for which he was treated over an extended period. The prosecutors said the treatment occurred before he was issued a pilot's license, and that they had found no indications that he was recently suicidal.

PHOTO GALLERY:Germanwings: The crash site

But Germany authorities have said that he had been issued multiple doctors' notes judging him unfit to work, including one covering the day of the plane crash. At least one of the notes was found torn up in his apartment. The system depends on employees reporting their own medical conditions to their employers, and Lufthansa has said that it was not aware of the recent medical problems.

An official familiar with the investigation said Tuesday that authorities were not examining the Lufthansa Group for any negligence. Lufthansa provided investigators with information about Lubitz's airline medical examinations and copies of previous correspondence with the airline, the official said. But since the depressive episode occurred in 2009, the official said that investigators did not believe Lufthansa was immediately culpable. During Lubitz's employment with Germanwings, starting in 2013, his medical certificates and examinations declared him flightworthy.

A Lufthansa spokeswoman said that the company had graduated him from its rigorous flight school, despite the previous depressive episode, because following medical checks "he was perceived to be healed."

"At any time he was flying, he was declared fit to fly," the spokeswoman said, who spoke on the condition that her name not be used, a German custom.

When asked whether Lufthansa had known about any subsequent psychological condition, she said: "Not that we are aware of."

Germany's medical examinations for pilots give a yes-or-no answer to employers about whether aviators are ready to fly, offering no space for additional information or caveats. Officials familiar with the investigation have said that one working theory is that Lubitz was concerned about losing his medical certificate when it came up for renewal later this year.

READ: Last minutes of ill-fated Germanwings flight

Michael Müller, chief executive of ATTC, a company that helps prepare pilot candidates for entering flight schools, including Lufthansa's, defended the carrier's track record. He said he was aware of at least one instance, for example, when the company had pulled a pilot from the cockpit after his ex-wife had committed suicide.

"I'm afraid it will never be possible to prevent these thing from happening entirely," he said. "In my view, Lufthansa did not fail. When a doctor says someone is healthy and he is certifying this, then he is allowed to fly. In a pilot's career, it can happen that you get ill, also psychologically. You can't simply say, 'We'll let him go.' "

The Lufthansa Group has already offered $53,635 to families of every victim to cover immediate living expenses. The new revelation was likely to open the airline to far greater damages. A Lufthansa spokesman said Tuesday that its insurer, Allianz, had set aside $300 million to pay for liability claims from victims' families.

French President François Hollande visited German Chancellor Angela Merkel in Berlin on Tuesday, where the two discussed the ongoing investigation into the catastrophe alongside a range of other issues.

Hollande called for bolstering the checks on pilots over European skies, saying that he was working toward "ensuring that we can strengthen our safety rules for piloting these aircraft."

He said that more than 800 people were laboring at the mountain crash site to push the investigation forward as quickly as possible.

Separately, a French aviation investigation agency said Tuesday that it had begun a study of "systemic weaknesses" that may have led to the crash. The French Bureau of Investigations and Analyses for Civil Aviation Security said it would focus on the procedures used "to detect psychological profiles," as well as look at cockpit safety rules.

German investigators offered few new details about the status of their inquiry on Tuesday. One official familiar with the investigation said the initial questioning of Lubitz's family and girlfriend had been completed, but that investigators remained in contact with them as new issues arose. The official said that neither Lubitz's parents nor his girlfriend were aware of any suicidal impulses ahead of the plane crash.