The black box voice recorder indicates that Andreas Lubitz, 27, locked his captain out of the cockpit on Tuesday and deliberately flew Flight 4U 9525 into a mountainside, French officials say, in what appears to have been a case of suicide and mass murder.
French Prime Minister Manuel Valls said that "everything is pointing towards an act that we can't describe: criminal, crazy, suicidal".
German prosecutors revealed that searches of Lubitz's homes netted "medical documents that suggest an existing illness and appropriate medical treatment", including "torn-up and current sick leave notes, among them one covering the day of the crash".
They did not specify the illness.
But Bild daily earlier reported that Lubitz sought psychiatric help for "a bout of serious depression" in 2009 and was still getting assistance from doctors, quoting documents from Germany's air transport regulator.
The paper also cited security sources as saying that Lubitz and his girlfriend were having a "serious crisis in their relationship" that left him distraught.
Lufthansa CEO Carsten Spohr said that Lubitz had suspended his pilot training, which began in 2008, "for a certain period", before restarting and qualifying for the Airbus A320 in 2013.
According to Bild, those setbacks were linked to "depression and anxiety attacks".
Lubitz lived with his parents in his small home town of Montabaur in the Rhineland and kept an apartment in Duesseldorf, the city where his doomed plane was bound.
Duesseldorf prosecutors said the evidence found in the two homes "backs up the suspicion" that Lubitz "hid his illness from his employer and his colleagues".
They said they had not found a suicide note, confession or anything pointing to a "political or religious" motive but added it would take "several days" to evaluate the rest of what was collected.
Reiner Kemmler, a psychologist who specialises in training pilots, noted that people "know that depression can compromise their airworthiness and they can hide it".
"If someone dissimulates, ie they don't want other people to notice, it's very, very difficult," Kemmler told Deutschlandfunk public radio.
- Desperate captain used 'axe' -
Lubitz locked himself into the cockpit when the captain went out to use the toilet, then refused his colleague's increasingly desperate attempts to get him to reopen the door, French prosecutor Brice Robin said.
According to Bild, the captain even tried using an axe to break through the armoured door as the plane was sent into its fatal descent by Lubitz.
This could not be immediately confirmed, but a spokesman for Germanwings told Bild that an axe was standard emergency equipment on board the aircraft.
The tragedy has already prompted a shake-up of safety rules, with several airlines announcing a new policy requiring there always be two people in the cockpit.
German aviation industry body BDL and the transport ministry agreed to the rule for Lufthansa, its subsidiary Germanwings and other companies, while the European Aviation Security Agency threw its weight behind the policy.
Meanwhile, the UN world aviation body stressed that all pilots must have regular mental and physical check-ups.
- 'Unimaginable' -
German Chancellor Angela Merkel said the findings that Lubitz appeared intent on crashing the plane added an "absolutely unimaginable dimension" to the tragedy, in which most victims were German and Spanish nationals.
In the northwestern town of Haltern, which lost 16 students and two teachers who were returning from a school exchange, the revelations prompted shock and rage.
The principal of the stricken school, Ulrich Wessel, said "what makes all of us so angry (is) that a suicide can lead to the deaths of 149 other people".
German President Joachim Gauck, a Protestant pastor, attended a memorial service in Haltern Friday and also extended special condolences to the families of the victims in Spain and other countries.
Meanwhile in Montabaur, Mayor Edmund Schaaf urged reporters encamped in the community to show restraint with Lubitz's parents, a banker and a church organist, who live in a handsome home on a leafy, normally quiet street.
"Regardless of whether the accusations against the co-pilot are true, we sympathise with his family and ask the media to be considerate," he said.
- Descent button -
Investigators say Lubitz's intention was clear because he operated a button sending the plane into a plunge.
For the next eight minutes, Lubitz was apparently calm and breathing normally.
"He does not say a single word. Total silence," Robin said.
The second-in-command had all psychological tests required for training, Lufthansa's Spohr told reporters Thursday, insisting: "He was 100-percent airworthy."
Recovery operations at the crash site were ongoing, with French officials trying to find body parts and evidence. A second black box, which records flight data, has not yet been recovered.
"There's not much plane debris left. There's mainly a lot of body parts to pick up. The operation could last another two weeks," said police spokesman Xavier Vialenc.