It is not clear exactly who any possible charges would target.
All 150 people on board, mostly from Spain and Germany, died in the crash in March.
Marseilles prosecutor Brice Robin said there was "no doubt" that co-pilot Andreas Lubitz deliberately crashed the Airbus A320 in the French Alps.
Both Germanwings and Lufthansa have previously said that Lubitz, 27, had passed all fitness to fly tests.
Lufthansa has also acknowledged that it knew the co-pilot had suffered from severe depression in 2009 while training for his pilot's licence.
Mr Robin said some doctors treating Lubitz felt he was unfit to fly but did not tell his employers because of German laws on patient confidentiality.
He said a preliminary investigation by three magistrates would focus on whether the gap between what the pilot's doctors knew, and what his employers knew, points to manslaughter charges.
Mr Robin said that Lubitz had seen seven separate doctors in the month before the crash - one GP, three psychologists and three eye specialists.
Lubitz was troubled about problems with his eyesight and just over a week before the crash, he told one doctor he was only sleeping two hours a night and feared he was going blind.
But doctors could find no "organic cause" for his failing sight, with one doctor suggesting that it might have been due to psychosis.
Mr Robin was speaking after meeting some of the relatives of those who died in the crash.
On Wednesday the coffins of 16 German schoolchildren and two teachers killed in the crash arrived in the town of Haltern.
Residents holding white roses lined the route as a convoy of white hearses passed the children's school.
The victims' remains were the first to be repatriated following delays over errors on the death certificates.
The remains of the rest of the victims will be repatriated over the coming weeks. The passengers were from 18 countries, including Australia, Argentina and Japan, but most of those on board were either Spanish or German.