Lead prosecutor Brice Robin had already announced on Friday that investigators were expanding their probe to manslaughter and appointed three judges to lead the investigation.
"The French penal code forbids me from opening a judicial enquiry for murder because the perpetrator is dead," said Robin at the time.
Investigators say that 27-year-old German co-pilot Andreas Lubitz intentionally downed the plane en route from Barcelona to Duesseldorf on March 24, killing all 150 on board, AFP reported.
It has since emerged that Lubitz had seen seven doctors in the month before the disaster.
Lubitz, who suffered from "psychosis", was terrified of losing his sight and consulted 41 different doctors in the past five years, including GPs, psychiatrists and ear, throat and nose specialists.
Several of these doctors who were questioned by German investigators said Lubitz complained he had only 30 percent vision, saw flashes of light and suffered such crippling anxiety he could barely sleep.
Lubitz reportedly said "life has no sense with this loss of vision".
However the doctors he consulted -- including one who booked him off work two days before the ill-fated flight -- did not reveal his mental struggles due to doctor-patient privilege.