French prosecutors have formally opened a criminal inquiry into Germanwings plane crash to investigate whether mistakes were made in monitoring the psychological health of the co-pilot.

Marseille prosecutor Brice Robin told a news conference in Paris on Thursday that the investigation will be led by a panel of three judges tasked to determine if manslaughter charges should be brought over the March 26 crash.

A preliminary report into the crash of the A320 in the French Alps that killed 150 people found co-pilot Andreas Lubitz locked the captain out of the cockpit and deliberately steered the plane into a mountain.

"The head of the Marseille court will appoint very soon, next week or the following week, three investigating judges from Marseille's collective accident centre after the opening of an inquiry against unknown parties for involuntary homicides," Robin said.

In a new development, Robin said information from Lubitz's tablet PC showed he had also investigated vision problems, and "feared going blind", which would have ended the 27-year-old's aviation career.

The news came as families have just started to receive the remains of their loved ones for burials in the coming days and weeks.

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", Robin told reporters.

Prosecutors have found evidence that Lubitz, who had suffered from severe depression, had researched suicide methods and concealed an illness from his employer, prompting airline industry executives to review pilot screening and licensing processes.

The French prosecutor added that the co-pilot had seen 41 doctors in five years, including seven during the month before the crash, but under Germany's stringent privacy laws the doctors could not report that to the airline.

Earlier in the day, the Marseille prosecutor and investigators met families of some of the victims behind closed doors in Paris, amid lingering questions about the co-pilot's actions and compensation issues.

Source: Al Jazeera And Reuters