"Military investigators have determined the person who activated, without permission and any particular reason, a fire safety system on board the submarine. He is a sailor from the crew, and he has already confessed," Vladimir Markin, a spokesman for the investigation at the Prosecutor General's Office said.
Criminal charges have already been brought against the crew member, and he faces up to seven years in jail.
The tragedy occurred late on Saturday while the Nerpa was undergoing sea trials in the Sea of Japan. Three submariners and 17 shipyard workers died in the accident. There were 208 people, 81 of them submariners, on board the vessel at the time.
Investigators earlier established that the fire safety system that was thought to have malfunctioned was in order.
The submarine's reactor was not affected by the accident, which occurred in the nose of the vessel, and radiation levels on board remained normal.
The incident is the worst for the Russian Navy since the sinking of the Kursk nuclear submarine in 2000 when all 118 sailors died.
The construction of the Akula II class Nerpa nuclear attack submarine started in 1991, but was suspended for over a decade due to a lack of funding. Akula II class vessels are considered the quietest and deadliest of all Russian nuclear-powered attack submarines.
The Nerpa started sea trials on October 27.