Dozens of crewmen were fast asleep on a nuclear submarine when freezing Freon gas poured over them from a firefighting system, survivors said Tuesday in the first eyewitness accounts of the Russian submarine accident that killed 20 people.

The Nerpa submarine was undergoing sea trials Saturday in the Sea of Japan when its fire-extinguishing system switched on, spewing liquefied Freon gas that asphyxiated the victims and sent 21 others to the hospital. The submarine returned to its home port under its own power Sunday.

Survivors said they were completely caught off guard.

"A siren blared and Freon came streaming in immediately," shipyard worker Viktor Rivk said on Russia's NTV television, speaking from a hospital bed in the Pacific coast port of Vladivostok. He looked feeble and exhausted, and the bridge of his nose was bruised.

Other survivors said a siren warning the crew the firefighting system was turning on may somehow have been delayed.

Seaman Denis Kashevarov told NTV the siren blared moments after the Freon came streaming in. He said a second wave of Freon came shortly afterward and then he passed out.

The daily Komsomolskaya Pravda quoted another survivor, Sergei Anshakov, who also said the siren went off only after gas began pouring in. A duty officer on the intercom then ordered everyone aboard to don breathing kits, Anshakov said.

Anshakov said some of the victims may have died because they were right under the stream of Freon, or they panicked, or they couldn't immediately reach their breathing kits.

Rivk said "practically everyone" in his section had breathing gear, but some may have failed to put it on quickly.

Officials blamed the accident — the worst on a Russian sub since the 2000 Kursk disaster killed 118 people — on a fire-safety system malfunction. But the state-run RIA-Novosti news agency cited an unidentified official on the government panel investigating the incident as saying Tuesday that the system was in working order.

He said the panel must determine whether there was a fire on board.

Navy spokesman Capt. Igor Dygalo said three was no fire and that the fire system was turned on in error, the Interfax news agency reported.

Navy experts have said that overcrowding and human error may have contributed to the accident.

The submarine had 208 people aboard when the accident occurred, including 81 seamen. The rest were civilians, many from the shipyard that built the submarine. Akula-class subs normally carry a crew of 73.

Former submariners say sea trials always pose increased safety risks, because of the large number of unqualified people on board. They say every person on board must permanently carry an oxygen kit, but many civilian workers may have lacked experience in using them.

Seventeen of the dead were civilians.

"I know from my own experience that instructions given to industrial personnel taking part in tests are a formality," the daily Komsomolskaya Pravda quoted retired Capt. Nikolai Markovtsev as saying. "Quite often they walk through a sub like they walk through their plant, without carrying breathing kits."

Breathing kits were available for everyone on the submarine, according to the official on the investigative commission.