A crew member was responsible for the deaths of 20 people on board a nuclear submarine in Russia's worst naval disaster for eight years, investigators announced today.

Prosecutors said that the unnamed submariner had switched on the Nerpa's fire extinguishing system "without permission or any particular grounds". Twenty died and 21 were injured after they inhaled freon gas released inside the vessel by the safety system.

"The suspect is one of the sailors of the crew, who for no reason set off the fire extinguishing system, as a result of which 20 people died and 21 were hospitalised," investigator Vladimir Markin told Vesti-24 television. He said that the man had already been charged and faced up to five years in prison.

The statement contradicted earlier assertions by investigators that the firefighting system on board the attack submarine had triggered automatically and that there was no evidence of human interference. They later ruled out any malfunction.

Critics of the investigation suggested that the sailor was being made a scapegoat. The submarine is due to resume sea trials as soon as the inquiry is complete.

Interfax news quoted an unnamed officer from the Nerpa's crew who expressed scepticism over the man's guilt. He said: "We have served together for a long time - since 2003. He is not green. He is a skilled specialist and it is not the first year that he is serving in the navy.

"We don't believe that he is guilty and fear that he could have confessed under pressure. He was taken away for interrogation yesterday and has still not come back."

An official responsible for overseeing policing activities also raised concerns. Anatoly Kucherena, head of the law-enforcement committee of Russia's Public Chamber, said: "It is certainly troubling that the investigative committee found the guilty party so quickly."

Survivors of the disaster told a Russian newspaper that gas masks issued to crew members had been faulty. Warrant officer Yevgeny Ovsyannikov told Tvoi Den: "I saw people in convulsions ripping off their masks. I also had a breathing apparatus on, but it only worked for seven to 15 minutes."

Dmitri Usachyov, another man on board, said: "Some of the dead were found with their gas masks on. The breathing apparatuses simply didn't work."

The tragedy occurred during sea trials in the Sea of Japan late on Saturday while the Nerpa was carrying 208 people, three times its normal number. Seventeen of the dead were civilian engineers involved in construction of the vessel at the Amur shipyard near Vladivostock in Russia's Far East.

A spokeswoman at the shipyard said that the submarine would resume testing once investigators had finished with it. Marina Radayeva said: "The Russian Navy has said it will still commission the submarine, therefore the sea trials will continue."

The accident was the worst for Russia's Navy since the explosion that sank the Kursk nuclear submarine and led to the deaths of all 118 crew in August 2000.

Russian and Indian media have reported that the Nerpa was due to be leased to the Indian Navy next August in a $650 million contract. Russian officials deny that an agreement has been reached.

The Akula II class vessel is regarded as one of Russia's quietest and deadliest submarines. Construction of the Nerpa began in 1991 but was postponed for more than a decade in the economic crisis that followed the collapse of the Soviet Union.

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