The fire safety system on a new Russian nuclear-powered submarine malfunctioned on a test run in the Sea of Japan, spewing chemicals that killed at least 20 people and injured 21 others, officials said Sunday.
It was Russia's worst naval accident since torpedo explosions sank another nuclear-powered submarine, the Kursk, in the Barents Sea in 2000, killing all 118 seamen aboard.
The victims died of poisoning from Freon gas that was released Saturday when the fire-extinguishing system accidentally turned on, said Sergei Markin, an official with Russia's top investigative agency.
His agency has launched a probe into the accident, which Markin said will focus on what activated the firefighting system. He suggested there could be possible violations of operating rules, which points to human error.
The submarine itself was not damaged and traveled back to its base on Russia's Pacific coast under its own power Sunday, Russian navy spokesman Capt. Igor Dygalo said.
The nuclear reactor that powers the sub was operating normally and radiation levels in the sub were also normal, Dygalo said.
The submarine returned to Bolshoi Kamen, a military shipyard and a navy base near Vladivostok, state-run Rossiya television said.
Dygalo said the deaths and injuries were due to the "unsanctioned activation" of the firefighting system in the two sections of the submarine closest to the bow.
Seventeen civilians and three seamen died in the accident and 21 others were hospitalized after being evacuated to a destroyer that brought them to shore, Markin said in a statement, revising earlier casualty figures.
Dygalo said the submarine had 208 people aboard, including 81 servicemen, and was to be commissioned by the navy later this year.
Officials did not reveal the name of the submarine, but Russian news agencies quoted officials at the Amur Shipbuilding Factory saying the submarine was built there and is called the Nerpa.
Construction of the Nerpa, an Akula II class attack submarine, started in 1991 but was suspended for years because of a shortage of funding, they said. Testing on the submarine began last month and it submerged for the first time last week.
First Deputy Defense Minister Alexander Kolmakov and navy chief Adm. Vladimir Vysotsky were heading for the Pacific Coast in the wake of the accident, Dygalo said.
Saturday's accident came as the Kremlin is seeking to restore Russia's naval reach, part of a drive to show off the nuclear-armed country's clout amid strained ties with the West. A naval squadron is headed to Venezuela for joint exercises this month in a show of force near U.S. waters.
Despite a major boost in military spending during Vladimir Putin's eight years as president, Russia's military is still hampered by decrepit infrastructure, aging weapons and problems with corruption and incompetence.
The Kremlin said President Dmitry Medvedev was told about the accident immediately and ordered a thorough investigation.
Putin, now prime minister, was criticized for his slow response to the Kursk disaster.
In 2003, 11 people also died when a Russian submarine that was being taken out of service sank in the Barents Sea.