Brice Robin, the prosecutor of the southern French city of Marseille, announced the next step in the probe to some 200 family members who flew to Paris for an update, said Stephane Gicquel, the head of the support group.
According to Gicquel, the judges would be investigating a case of "manslaughter".
Investigators say that 27-year-old German co-pilot Andreas Lubitz intentionally downed the plane en route from Barcelona to Duesseldorf on March 24 in the crash that left 150 people dead.
He said Lubitz had consulted "more than 40 doctors and notably five or six in the month" before the tragedy.
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Gicquel said the families were shown three different reconstructions of what had happened in the cockpit.
Some families were left outraged when Lufthansa informed them that repatriation would be delayed due to problems with the issuing of death certificates.
The mayor of the French village of Prads-Haute-Bleone, near the crash site, said there had been slight spelling errors "of foreign-sounding names" on several death certificates.
After a complaint by the families of some schoolchildren killed in the crash, who had already planned funerals, a flight returning their remains went ahead as planned yesterday.
However to date only 44 Germans out of the 150 people killed in the March 24 disaster have been returned to their home country for burial.
Lufthansa, the parent company of budget airline Germanwings, has vowed that French authorities were working hard to resolve administrative formalities "as soon as possible".