The documents, which were found in his home, included torn-up and current sick leave notices from a doctor, including one covering the day of the crash declaring him "unfit to work".

"We have found a letter that indicated that he was declared by a medical doctor 'unfit to work'," said Dusseldorf Prosecutor Christoph Kumpa. "[It] was found slashed in a dust bin, so we have reason to believe he hid his illness from the company he was working for." 

German prosecutors outlined details of their findings in a statement from their office in Duesseldorf, where the co-pilot lived and where the doomed flight from Barcelona was heading, 

"Documents with medical contents were confiscated that point towards an existing illness and corresponding treatment by doctors," German prosecutors said.

"The fact there are sick notes saying he was unable to work, among other things, that were found torn up, which were recent and even from the day of the crime, support the assumption based on the preliminary examination that the deceased hid his illness from his employer and his professional colleagues." 

They found no suicide note or confession, "nor was there any evidence of a political or religious background to what happened."

The statement came after German newspaper Bild reported Lubitz had been treated for at least one "serious depressive episode" in the past and may have tried to hide psychological troubles.

The account also cited security sources saying Lubitz has been in a "life crisis" that included troubles with his girlfriend.

It said Lubitz had slowed his flight training because of treatment for unspecified psychological issues, and was temporarily deemed "unfit to fly" during instructions at Lufthansa's aviation school outside Phoenix.

Lubitz's personnel file contained a special code saying he needed to have "special regular medical examinations," Bild added.

The report drew no direct connection between Lubitz's present psychological state and the crash. 

Investigators have been combing through his background, including whether he might have had a history of depression or psychological problems. They are also looking into his family background, and whether he had financial troubles or difficulties in his personal relationships.

Lubitz's mental health - and what Germanwings and parent company Lufthansa knew about it - could become central questions in any future legal case over the crash. Under German law, employees are required to inform their employers immediately if they are unable to work.

Lubitz, 27, is believed to be responsible for slamming Germanwings Flight 4U9525 into a mountainside in the French Alps on purpose, killing all 150 on board, while en route from Barcelona, Spain, to Düsseldorf. Prosecutors are examining several theories, including that the crash was a suicide or a mass murder.

On Thursday, the French prosecutor leading the investigation said the evidence from the cockpit voice recorder suggested that Lubitz, a former flight attendant with a passion for flying, had locked the pilot out of the cockpit and deliberately set the plane on its lethal descent.

The crash claimed victims from more than a dozen countries, including Germany, Spain and the United States.

Carsten Spohr, the chief executive of Lufthansa, the parent company of Germanwings, said Lubitz had passed the company's health checks with "flying colours".

"He was 100 per cent flightworthy without any limitations," Mr Spohr said.

But he said there had been an instance six years ago when Lubitz took a break from his training for several months. He said that if the reason was medical, German rules on privacy prevented the sharing of such information. Spohr said the revelation of Lubitz's actions had left him stunned.

Some international airlines responded to the crash by introducing new rules requiring that two crew members always be present in the cockpit, after the French prosecutor revealed that Lubitz had locked the plane's pilot out of the cockpit before starting the deadly descent. The airlines that said they were instituting a two-person rule in the cockpit included Air Canada, easyJet and Norwegian Air Shuttle.

All German airlines will introduce that requirement, the German aviation association said on Friday.

Thomas Winkelmann, the head of Germanwings, however, expressed doubt that such a rule would have prevented Tuesday's crash.

"I ask myself, when a person is so bent on committing a criminal act, whether that is preventable if for example a stewardess or steward is in the cockpit," Mr Winkelmann told the German public broadcaster ZDF.

"The suffering and pain this catastrophe has caused is immeasurable," he was quoted as saying in a message posted on Twitter by Germanwings on Friday. "No words can express it and no amount of consolation is sufficient."

Investigators are still trying to understand why the pilot left the cockpit, although most airlines allow it during noncritical phases of flight. There are no regulations requiring that a second crew member be present in the cockpit when one pilot leaves, usually for physiological reasons. The French prosecutor, Brice Robin, said it was reasonable to assume the pilot left the cockpit to use the toilet.

Members of a flight crew would typically use a numeric code to open the door if someone in the cockpit could not or would not let them in. The pilot would have known the code, Spohr said. However, the co-pilot could have activated a switch that prevents the door from opening for five minutes, or he could have found some other way to block the door, Spohr said.

Robin said that the Germanwings flight had begun prosaically, with polite exchanges between the two pilots as the flight began its course to Düsseldorf.

However, about a half-hour into the flight, he said that Lubitz appeared to have locked out the pilot of the plane and did not let him back in, prompting the pilot to demand access. Investigators, citing the plane's voice recorder, said they could hear the sound of someone trying to break down the door.

Robin said the plane's voice recorder showed that Lubitz was breathing normally in the moments leading up to the crash, indicating that he had deliberately crashed the plane.

The New York Times, Washington Post, AFP, Reuters

smh.com.au