German pilot Andreas Lubitz consulted doctors and medical specialists 41 times in the five years before crashing Germanwings Flight 9525 into the Alps, killing 150 people on board.

In the month leading up to the crash, Lubitz consulted doctors seven times, visiting a general practitioner, a psychiatrist three times, and three times to a nose, ear and throat specialist, Marseille prosecutor Brice Robin told reporters.

A number of doctors treating the pilot felt he was unfit to fly, but did not tell his employers because of German medical confidentiality laws.

Some felt that Lubtiz was psychologically unfit to fly, but "unfortunately that information was not reported because of medical secrecy requirements".

In Germany, doctors can be jailed if they disclose medical information about patients unless they have reason to believe the person is a threat to themselves or others.

Lubitz, 27, consulted 41 doctors in total in the years leading up to the crash, telling one of his doctors that he had consulted numerous neurologists and eye specialists, said Robin.

According to the prosecutor, Lubitz feared he was going blind, and suffered severe depression involving psychosis and vision problems.

He told people close to him that "life no longer had any meaning considering the loss of his eyesight", Robin said.

To doctors, he complained that he saw objects incompletely in the dark, experienced lights flashes and could not sleep as a result of his vision problems.

In the wake of the crash on March 24 northwest of Nice, investigators searching Lubitz's Munich flat found prescription drugs to treat depression and anxiety, as well as sick notes from medics, including one for the day of the fatal crash.

Robin said on Thursday that French authorities are opening a criminal investigation into the crash.

Investigators believe that Lubitz deliberately crashed the plane after researching suicide methods and ways of securing the cockpit against entry, but others could face criminal charges as part of the investigation.

Germanwings said in a statement that it can't comment on the prosecutor's findings, as Lubitz's medical records are subject to privacy laws.