It has been revealed that GermanWings co-pilot Andreas Lubitz, 27, could have been struggling with relationship issues with his girlfriend and that the pair could have recently broken up.

The Daily Mail reported that Lubitz had recently purchased two Audis, one for himself and one he was going to give to his girlfriend. The name of the girlfriend has not been disclosed.

Lubitz had previously suffered from mental illness and depression, and German authorities found torn-up sick notes showing that he was suffering from an illness that should have grounded him on the day of the tragedy. He apparently hid this from the airline.

French prosecutors believe Andreas Lubitz locked himself alone in the cockpit of the Germanwings Airbus A320 on Tuesday and deliberately crashed the plane into a mountain, killing all 150 people on board.

"Documents with medical contents were confiscated that point towards an existing illness and corresponding treatment by doctors," said the prosecutors' office in Duesseldorf, where the co-pilot lived and where the Germanwings Airbus A320 flight from Barcelona was heading.

"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," the German prosecutors said.

No suicide note or confession was found at his home, or any evidence of "a political or religious background to what happened."

German media suggested that Lubitz had suffered from depression in the past, and that Lufthansa would have been aware of at least some of that history.

Germany's Bild newspaper reported that Lubitz had suffered from depression during a period when he stopped his training six years ago, and had spent over a year in psychiatric treatment.

Lufthansa has acknowledged Lubitz broke off his training in 2009, but stated that there was nothing to suggest that he was at risk after he was cleared.

"After he was cleared again, he resumed training. He passed all the subsequent tests and checks with flying colors," Lufthansa CEO Carsten Spohr said on Thursday.

Bild cited internal documents forwarded by Lufthansa's Aero Medical Center to German authorities, reporting that Lubitz had suffered from depression and anxiety, and had been judged to have suffered a "serious depressive episode" around the time he suspended his training.

Lufthansa and German prosecutors declined to comment on the report.

Reuters contributed to this report.