The investigation will determine whether or not manslaughter charges can be brought over the crash, according to the BBC.
A preliminary report into the crash of the A320 in the French Alps found co-pilot Andreas Lubitz locked the captain out of the cockpit and deliberately steered the plane into a mountain.
Marseille Prosecutor Brice Robin said at a news conference in Paris the investigation will be lead by a panel of three judges tasked with determining if mistakes were made in analysing the mental state of Lubitz.
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.
In May, a preliminary report confirmed a growing picture of painstaking preparations carried out by Lubitz before the crash. The European Commission has asked the European Aviation Safety Agency to assess the report and will then decide whether to update aviation safety rules.
The French prosecutor said German investigators had confirmed that Lubitz had searched online for drugs such as diazepam, for ways to kill himself and for sight problems.
On Thursday, German newspaper Sueddeutsche Zeitung and broadcasters NDR and WDR cited an analysis of a computer found in Lubitz`s Düsseldorf apartment showing he had searched the Internet for ways to buy potassium cyanide, Valium and lethal drug cocktails in March.
The day before the crash, Lubitz had researched living wills, which detail a person`s wishes related to medical treatment that extends life, on the website of the Hamburg Medical Chamber, the media reports said. They said this was likely out of fear that his suicide attempt could fail.
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