The source also said the submarine had the required amount of individual breathing kits on board, adding however that it was too soon "to make any final conclusions as to the cause of the deaths."
"The main question now is whether or not there was a fire on board the submarine in the first place," he said.
The tragedy occurred late on Saturday while the Nerpa nuclear-powered submarine 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 said the deaths were caused by suffocation after Freon gas was released following the activation of the submarine's fire safety system. It has been suggested that human error could have been a factor in the accident.
Russian Navy spokesman, Capt. 1st Rank Igor Dygalo, has said that the submarine's reactor was not affected by the accident, which occurred in the nose of the vessel, and that radiation levels were 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.