The co-pilot who crashed a Germanwings jet into the French Alps feared he was losing his eyesight, and doctors he consulted felt he was unfit to fly, a French prosecutor said.

An investigation into the March 24 plane crash which killed all 150 people on board – including Melbourne nurse Carol Friday, 68, and her 29-year-old son Greig – continued Thursday.

Marseille Prosecutor Brice Robin told reporters that 
doctors didn’t report their concerns to co-pilot Andreas Lubitz’s employers, because of German patient privacy laws.

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Mr Robin said the investigation so far “has enabled us to confirm without a shadow of a doubt … Mr Andreas Lubitz deliberately destroyed the plane and deliberately killed 150 people, including himself.”

Investigators said Lubitz locked the pilot out of the cockpit and flew the plane into a French mountainside, after having researched suicide methods and cockpit door rules and practised an unusual descent.

Victorians Carol Friday and son Greig were on board the fatal Germanwings flight.

Mr Robin said Lubitz had also investigated vision problems and “feared going blind” – a career-ending malady for a pilot.

He had suffered depression in the past and was on anti-depressants.

The investigation revealed Lubitz went to seven medical appointments within the month before crash, including three appointments with a psychiatrist, Mr Robin said.

“Some of the doctors felt Lubitz was psychologically unstable, and some felt he was unfit to fly, but “unfortunately that information was not reported because of medical secrecy requirements,” he said.

In Germany, doctors risk prison if they disclosed information about their patients to anyone unless there was evidence they intendee to commit a serious crime or harm themselves.

Germanwings and parent company Lufthansa said that Lubitz had passed all medical tests and was cleared by doctors as fit to fly.

The question for investigators now was who could be held responsible.

The prosecutor upgraded the investigation from a preliminary probe to a full-fledged manslaughter inquiry, which handed the case to investigating magistrates who could file eventual charges against people or entities.

German lawyer Peter Kortas, whose firm represented relatives of 34 victims, said negotiations with Germanwings about compensation began several days ago.

Families were also seeking answers about delays in the return of victims’ remains.

– with AAP