The tragedy occurred late on November 8 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.
"We submariners are unanimous: a computer program failed. Previously, the submarine fire suppression system had always started manually on the commander's orders. Now it is launched electronically," Ensign Yevgeny Ovsyannikov, a technical specialist on the Nerpa, told Komsomolskaya Pravda.
He added that it was the first time this computerized system had been used on the submarine during the sea trials and that the computer had malfunctioned during tests in the dock.
An expert who requested anonymity suggested that a toxic form of Freon could have been used in the fire suppression system.
"A toxic additive, trichlorotrifluoroethane [C2F3Cl3], was used. It is cheaper than pure Freon. Possibly, they simply wanted to economize," he said.
He added that there were unmistakable signs of poisoning, which could not have been caused by Freon: "People were collapsing as though they had been shot."
Breathing Freon is generally safe, but if the concentration in the air is high then suffocation can result.
He said 46 people had been hospitalized, not 21 as officially announced.
It was previously reported that the deaths were caused by a crew member activating the fire safety system without permission or by the wrong data being entered into the temperature sensor.
Submariner Dmitry Grobov is suspected of having entered the wrong temperature data for the submarine's living quarters, which caused the fire suppression system to release the Freon gas.
However, former Navy officers have said they doubt that Grobov was solely to blame since it is impossible for one person to activate the system, which is security protected from unauthorized activation by multiple authentication levels.
The submarine's nuclear reactor was not affected by the accident, which took place in the nose section, and radiation levels on board remained normal.
Investigators earlier announced that they had brought criminal charges against the crew member, and that he faced up to seven years in jail if found guilty.
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.
Based in the Russian Far Eastern city of Komsomolsk-on-Amur, the Amur Shipyard has built 270 vessels, including the Nerpa and another 55 nuclear submarines since it was established in 1936.