Twenty people died aboard the Russian nuclear submarine Nerpa after a fire safety system was activated without permission, investigators said Thursday.

Criminal charges have been brought against a member of the crew, who confessed, spokesman Vladimir Markin said. The suspect faces a possible seven years in jail, RIA Novosti reported.

The event occurred late Saturday while the Nerpa was undergoing trials in the Sea of Japan. Three sailors and 17 shipyard workers died in the accident. There were 208 people, 81 of them sailors, on board at the time.

The submarine's reactor wasn't affected and radiation levels on board remained normal. Sea trials will continue after the investigation is completed, officials told RIA Novosti.