BERLIN -- Lufthansa knew six years ago that the co-pilot of the passenger plane that crashed in the French Alps last week had suffered from a "serious depressive episode," the German airline said Tuesday.
The airline said that as part of its internal research it found emails that Andreas Lubitz sent to the Lufthansa flight school in Bremen when he resumed his training there after an interruption of several months.
In them, he informed the school that he had suffered a "serious depressive episode," which had since subsided.
The airline said Lubitz subsequently passed all medical checks and that it has provided the documents to prosecutors. It declined to make any further comment.
The revelation that Lufthansa officials had been informed of Lubitz's psychological problems raises further questions about why he was allowed to become a pilot for its subsidiary, Germanwings, in September 2013.
Authorities say the 27-year-old Lubitz, who in the past had been treated for suicidal tendencies, locked his captain out of the cockpit before deliberately crashing the Airbus 320 into a mountain in the French Alps on March 24. All 150 people aboard Flight 9525 from Barcelona to Duesseldorf were killed.
Earlier Tuesday, Lufthansa said it had set aside $300 million to deal with possible costs from the crash as French aviation investigators said they were examining "systemic weaknesses" like cockpit entry rules and psychological screening procedures that could have led to the Germanwings plane crash - issues that could eventually change worldwide aviation practices.
French aviation agency BEA signaled the latest re-think about airline procedures in the wake of the Germanwings crash, which jolted an aviation industry already reeling after one passenger plane disappeared into an ocean and another was shot out of the sky over war-torn eastern Ukraine.
The goal of the BEA investigation is to make recommendations to aviation authorities, not just in France but anywhere, about what can be done to prevent similar crashes. French prosecutors are carrying out a separate crash probe to pinpoint possible criminal wrongdoing.
The Germanwings crash has already produced some changes in aviation procedures. Europe's aviation regulator now says all airlines in Europe should require two people in the cockpit at all times during a flight. Many airlines have already imposed the new rule, which has been in place in the U.S. since the Sept. 11 attacks.
The International Civil Aviation Organization, which brings together 191 nations, said state agencies like the BEA must officially determine the causes and contributing factors of crashes and give recommendations on ways to avoid recurrences. ICAO could then bring such recommendations to its member states - possibly leading to changes in international aviation standards.
BEA said it aims to provide a "detailed analysis" of the Germanwings cockpit voice recorder and any other flight data - but it also plans to widen its search, to examine issues that could be problematic for all airlines.
"(We will study) systemic weaknesses (that) might possibly have led to this aviation disaster," BEA said in its first statement since prosecutors detailed the co-pilot's suspected role in the crash.
The agency is studying both psychological screening procedures and rules applied to entering and leaving the cockpit, as well as cockpit door locking systems.
German prosecutors say Lubitz received psychotherapy before obtaining his pilot's license and that medical records from that time referred to "suicidal tendencies." They have given no dates for his treatment, but said visits to doctors since then showed no record of any suicidal tendencies or aggression against others.
They also have found torn-up sick notes from doctors, including one that would have kept Lubitz off work on the day of the crash.
All pilots must undergo regular medical checks that include a cursory psychological evaluation, according to Dr. Hans-Werner Teichmueller, the agency's head. But such tests rely on patients being honest with their doctors, and even a seriously mentally unstable person would have been able to put a "mask" on for the investigation, he said.
"A pilot can hide mental problems and psychological problems and you cannot see what is behind his face," Teichmueller told CBS News' Allen Pizzey.
"We have developed a very refined system in Europe and most of us are in agreement that this system is optimal," he said in a separate interview. "If we were to add more psychological tests or modify the way we test, then we can still not change a situation like this."
At the crash site in the French Alps, investigators said they hope to have found DNA samples for everyone killed on the flight in the next 24 hours. Lt. Col. Jean-Marc Menichini, speaking in the town of Le Vernet, said the search was still on for the plane's second black box - its data recorder.
"By the end of the week at the latest, it will be possible to identify all the victims thanks to the DNA samples taken," French President Francois Hollande told reporters during a trip to Germany.
Hollande said German and French ministers also discussed the need to improve checks of air passengers within Europe's visa-free Schengen travel zone and to "ensure that we can also strengthen our safety rules for piloting planes."
Construction workers have cut a road up to the steep, mountainous crash site to speed up recovery efforts. Previously, emergency workers had to rely on helicopters. German investigators tasked with identifying the victims and determining their cause of death are expected Wednesday at the crash site.
In Frankfurt, Lufthansa spokeswoman Kerstin Lau said insurers have reserved $300 million to deal with "all costs arising in connection with the case."
Lufthansa - Germanwings' parent company - offered immediate aid last week of up to 50,000 euros ($54,250) per passenger to relatives of the victims. Those payments are separate from eventual compensation payments.
Airlines on international flights are required to compensate relatives of victims for proven damages of up to a limit of about $157,000 - regardless of what caused the crash. However, higher compensation is possible if a carrier is held liable.
Lufthansa has canceled plans to celebrate its 60th anniversary on April 15 "out of respect for the victims of the crash."