The man suspected of deliberately crashing a Germanwings A320 plane in the French Alps required treatment for depression, German media say.
Regular assessments were recommended in Andreas Lubitz's official notes after a serious episode some years ago.
The Barcelona-Duesseldorf plane crashed on Tuesday, killing 150 people.
Data from the plane's voice recorder suggest Mr Lubitz purposely started a descent as the pilot was locked out of the cockpit.
Several airlines have now pledged to change their rules to ensure at least two crew members are present in the cockpit at all times.
Police have searched two German properties used by Mr Lubitz, taking away boxes and a computer.
When Mr Lubitz finished training in 2009, he was diagnosed with a serious depressive episode and went on to receive treatment for a year and a half, the German news site Bild reports.
Internal documents quoted by Bild and German broadcaster ARD say a note on Mr Lubitz's aviation authority file recommended regular psychological assessment.
Andreas Lubitz: Germanwings co-pilot under scrutiny
- Started training in 2008, at Bremen and Arizona. Training was interrupted for some months - but he later passed all tests and was deemed fit to fly
- Working as co-pilot, or first officer, since 2013. Appeared pleased with his job
- Lived in town of Montabaur, near Frankfurt, reportedly with his parents. Kept a flat in Duesseldorf and had many friends
- Facebook profile suggests the active lifestyle of a keen runner, with an interest in pop music
Mr Lubitz's employers have confirmed that his training was interrupted for several months six years ago.
But they have not said why. Carsten Spohr - the head of Lufthansa, the German carrier that owns Germanwings - said on Thursday that Mr Lubitz was only able to resume training after his suitability was "re-established".
"He passed all the subsequent tests and checks with flying colours," Mr Spohr was quoted as saying.
German media are also reporting that investigators have found evidence of mental health problems at Mr Lubitz's Duesseldorf flat.
Earlier, another media report quoted a police spokesman as saying "a very significant clue as to what has happened" had been found during the search of the house the 27-year-old shared with his parents in Montabaur, north of Frankfurt, without specifying what.
Police said the discovery was not a suicide note.
French Prime Minister Manual Valls said the investigation was ongoing, but that "everything is pointing to a criminal, crazy, suicidal action that we cannot comprehend".
He said investigators and Lufthansa would have to "shed light on the career and profile of this pilot".
Police continued to come and go at the Montabaur house throughout Thursday morning, reports the BBC's Anna Holligan who is outside.
She says there are concerns for Mr Lubitz's parents, who have suffered not just one trauma - their son dying in a crash - but the subsequent shock of finding out he may have been responsible for the tragedy.
Based on data from the recovered "black box" voice recorder, Marseille prosecutor Brice Robin said on Thursday that Mr Lubitz appeared to want to "destroy the plane".
The evidence suggested the pilot had left the cockpit, probably to go to the toilet, during which the door was barred. He fought unsuccessfully to get back in, he said.
Bild quotes security sources as saying that the pilot used an axe to try to break down the door.
A Germanwings spokesperson would only confirm that an emergency axe is part of the equipment on board an A320.
Family members of some of the 150 passengers and crew who died have visited Seyne-les-Alpes, near the crash site, reported Reuters news agency.
They were accompanied by psychologists, paramedics and Red Cross workers, and a youth centre in the town was set up to receive them, it said.
Families are providing DNA samples to allow for identification of victims' remains.
Meanwhile, investigators continue to comb the crash site for body parts, debris and the second "black box", which records flight data and still has not been found three days after the plane crashed.
Members of the Westerwald flight club, where the co-pilot was a member, expressed their shock at the revelations about Mr Lubitz's mental history.
"Andreas was a very nice young man, who did his training here. He was part of the club," Peter Reucker said.
"[He was] funny, sometimes a bit quiet, but apart from that a young man like many others that we have here. He integrated well.
"I'm absolutely speechless. I have no explanation for this," Mr Reucker added.
Crash site close-ups
Other incidents thought to be caused by deliberate pilot action
- 29 November 2013: A flight between Mozambique and Angola crashed in Namibia, killing 33 people. Initial investigation results suggested the accident was deliberately carried out by the captain shortly after the first officer (also known as the co-pilot) had left the flight deck.
- 31 October 1999: An EgyptAir Boeing 767 went into a rapid descent 30 minutes after taking off from New York, killing 217 people. An investigation suggested that the crash was caused deliberately by the relief first officer but the evidence was not conclusive.
- 19 December 1997: More than 100 people were killed when a Boeing 737 travelling from Indonesia to Singapore crashed. The pilot - suffering from "multiple work-related difficulties" - was suspected of switching off the flight recorders and intentionally putting the plane into a dive.
Source: Aviation Safety Network