The investigators said several of the doctors Lubitz visited, as well as seven he had booked appointments with in the month before the accident, felt he was unfit to fly.
But they did not share their concerns with the 28-year-old's employer, Lufthansa, due to Germany's medical secrecy requirements.
In Germany medical practitioners can be sent to prison if they disclose patient information without evidence they intend to commit a serious crime or harm themselves.
Lead investigator Brice Robin said Lubitz had sought advice for potentially career-ending vision problems, and had sent an email to one doctor just two weeks before the accident saying he had doubled his dose of antidepressants in an attempt to help him sleep.
Robin said they could not confirm whether Lubitz's sight problems were real or psychological.
Germanwings and its parent company Lufthansa say the young pilot had passed all medical tests and was considered fit to fly. It has been confirmed that Lubitz had been struggling with depression and suicidal tendencies when Flight 4U9525 crashed.
Investigators met with relatives of the 150 victims of the crash to update them on the inquiry's progress. The victims' remains are currently being returned to families, and funerals will be held in the coming weeks. The remains of all those on board were finally identified last month.
Stephane Gicquel, head of a French accident victims association, welcomed the news that the inquiry was being expanded into a criminal investigation. "We can clearly see the prosecutor's positioning: to open an inquiry that will pose the question of manslaughter and, very clearly, faults or negligence from Lufthansa in detecting the state of Lubitz's health."
Investigators believe Lubitz locked the cockpit door after the pilot left, and put the plane into a controlled descent. During Thursday's meeting relatives were shown three different reconstructions of what happened in the cockpit, with actual audio from the fatal flight. "I think that the families appreciated this frank and direct communication," Gicquel said.
Around half of those who died in the crash were German, as well as victims from countries including Australia, Spain, the United States, Belgium and the United Kingdom.
an/bk (AFP, dpa, AP, Reuters)