Der Spiegel reported that prosecutors had found ripped-up pieces of paper and medical documents at the Düsseldorf apartment of Andreas Lubitz, who was alone at the controls of the plane at the time of the crash after locking his captain out of the cockpit. One of the documents was a doctor’s note excusing him work for Tuesday, the day of the crash.

Prosecutors said the find supported the thesis “that the deceased had hidden his illness from his employer and his those around him at work.”

German media outlets have put together anecdotal evidence that Lubitz had a history of suffering from depression, identifying it as the reason he broke off his pilot training in 2008 for 18 months. He was subsequently cleared to fly in 2010 by Germany’s Federal Aviation Office.

“During his training at Lufthansa Flight School, Andreas L was listed as unsuitable for flight duties because he spent one and a half years in psychological treatment and so had to sit some courses again. The reason was evidently depression,” an unidentified source told the mass-circulation Bild-Zeitung.

The Wall Street Journal, meanwhile, quoted Holger Kasperski, a spokesman for the FAO, as saying that “we have a pilot license and a medical certificate,” but couldn’t say whether the condition related to Lubitz’s physical or mental health.

The revelations may increase the pressure on Germanwings’ parent company, Deutsche Lufthansa AG (DLAKY), to explain why Lubitz was allowed to fly an airliner. Chief executive Carsten Spohr told a press conference Thursday that Lubitz was “100% fit to fly”, and had passed the regulation tests. The FAO had declared Lubitz fit to fly in 2010.

A spokesman for Lufthansa told Fortune that “it isn’t unusual” for people to interrupt flight training, and that the FAO is forbidden by law from divulging details of medical tests to employers, as a matter of patient-doctor confidentiality. He said that trainee pilots are only screened once at the start of their training for mental illness, and that the company relies on the observations of colleagues on a day-to-day basis to pick up signs of such problems. That system is now looking alarmingly inadequate.

Bild reported that Lubitz had had a “serious depressive episode” at the time he broke off his training. The Frankfurter Allgemeine Zeitung said Lubitz had admitted to “burnout” and “depression” to a female friend some years ago, according to her mother.

Reuters quoted another friend of Lubitz who had met him six years ago as saying that the young man had become increasingly withdrawn over the past year.

“He always used to be a quiet companion, but in the last year that got worse,” Reuters reported Lubitz’s friend as saying.

Germany’s largest airlines, including Lufthansa and its Germanwings and Condor subsidiaries, agreed on Friday to introduce the so-called “rule of two” immediately, a spokeswoman for the industry association said. A number of other airlines, including Norwegian Air Shuttle, Britain’s easyJet, Air Canada and Air Berlin, said that they would do likewise already on Thursday. The rule, which is already in force in the U.S. but not in Europe, ensures that a pilot is never left alone in the cockpit.