The tragedy, the worst since the sinking of the Kursk in 2000, 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 - 208 people, 81 of them submariners, had been on board the vessel.
"The preliminary official conclusion is the unauthorized operation of the fire extinguishing system," Ksenia Gusentsova, spokeswoman for the chief federal inspector for the Primorye Territory, Russia's Far East, told RIA Novosti.
The governor of the neighboring Khabarovsk Region said the investigation revealed that the fire safety system had operated correctly.
Earlier Capt. 1st Rank Igor Dygalo, an aide to the Russian Navy commander, said that the submarine's reactor had not been affected by the accident, which occurred in the nose of the vessel, and radiation levels were normal.
Meanwhile, Khabarovsk Governor Viktor Ishayev said on Monday the investigation suggested that human error could have been a factor in the deaths.
"The ship has the latest fire-extinguishing system, a computer-activated alarm" Ishayev said. "According to the investigation, there is no evidence that a person was involved in the system's operation."
Investigators said the deaths were caused by suffocation after Freon gas was released following activation of the submarine's fire safety system.
He added that those working in the area that had donned breathing apparatus had survived, while those who failed to use safety equipment had all perished.
Gennady Illarionov, a former naval submariner, told RIA Novosti, "You can't rule out that some of the civilians had not been issued with portable breathing apparatus or didn't know how to use them,"
And a high-ranking source at the Pacific Fleet's headquarters also questioned why portable breathing apparatus had not been used, "Perhaps the submariners did not notice the gas being released and when they realized it was already too late."
Tuesday has been declared a day of mourning in Primorye Territory, where the Russian Navy's Pacific Fleet is based.
The incident is the worst for the Russian Navy since the sinking of the Kursk submarine in 2000 when all 118 sailors onboard died.
The construction of the Akula II class Nerpa nuclear attack submarine started in 1991 but was suspended for over a decade due to lack of funding. Akula II class vessels are considered the quietest and deadliest of Russian nuclear-powered attack submarines.
The Nerpa started sea trials on October 27.
Gusentsova said the accident investigation commission included the commander-in-chief of the Navy, Adm. Vladimir Vysotsky, the first deputy defense minister, Gen. Col. Alexander Kalmykov, the chief federal inspector for the Primorye Territory, Sergei Lelyukhin, as well as investigators from the Russian Prosecutor General's Office.
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