The Germanwings co-pilot who flew his Airbus into a French mountainside, killing all 150 aboard, suffered serious depression, a German newspaper reported Friday, raising new questions over how he was cleared to fly.

The black box voice recorder indicates that Andreas Lubitz, 27, locked his captain out of the cockpit on Tuesday and deliberately sent Flight 4U 9525 into the Alps, French officials say, in what appears to have been an act 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."

Neighbours and colleagues described the co-pilot as a well-liked man, a fitness fanatic who lived with his parents in a leafy, upscale street in the west German town of Montabaur.

But a troubled man apparently hid behind that guy-next-door image.

Lubitz sought psychiatric help for "a bout of serious depression" in 2009 and was still getting assistance from doctors, Bild daily said, quoting documents from Germany's air transport regulator Luftfahrtbundesamt (LBA).

RELATED STORIES:
French investigators hunt black box from doomed Germanwings jet
Germanwings co-pilot Andreas Lubitz suffered depression: report

He was still receiving regular medical treatment, Bild reported, adding that Germanwings' parent company Lufthansa had transmitted this information to the LBA.

The paper also cited security sources as saying that Lubitz and his girlfriend were having a "serious crisis in their relationship" that left him heartbroken and 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".

German police combed for clues in an apartment Lubitz used in Duesseldorf, but spokesman Marcel Fiebig told AFP there was no "smoking gun".

Searches were also carried out at his parents' house. The street was cordoned off Thursday as officers wearing gloves emerged with boxes, bags and briefcases but was open again to traffic Friday.

News website Spiegel Online said that investigators had unearthed unspecified evidence in the Duesseldorf flat that Lubitz suffered from psychological problems.

In the latest report, German prosecutors has reportedly said the co-pilot hid sick leave note for crash day.