Moscow - A careless crew member caused a weekend accident on a Russian nuclear submarine that cost 20 lives, an inquiry showed Thursday.

The inquiry established that a sailor set off the "anti-fire system on board the submarine without authorization and for no reason," the Itar-Tass news agency quoted a prosecution spokesman as saying.

Twenty people suffocated when the fire-extinguishing system on board the new Akula-class submarine Nerpa released a freon gas coolant into the vessel Saturday as it underwent a test run in the Sea of Japan.

The sailor, who confessed his error, faces up to seven years in prison.

It was the Russian Navy's worst accident since all 118 sailors aboard the Kursk nuclear-powered submarine died when one of its torpedoes exploded, sinking the craft in the Barents Sea in 2000. (dpa)