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 suffocated Saturday after the submarine's fire-extinguishing system accidentally turned on and released Freon gas, said Sergei Markin, an official with Russia's top investigative agency. He said forensic tests found Freon in the victims' lungs.
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. He said the accident affected 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.
Lev Fyodorov, a top Russian chemical expert, said Freon pushed oxygen out, causing those inside to die of suffocation. He also said the scarce official information was making it difficult to understand exactly what happened on the submarine.
It wasn't immediately clear why personnel affected failed to activate the individual breathing kits they were supposed to have, he said.
Markin's agency has launched a probe into the accident, which he said will focus on what activated the firefighting system and possible violations of operating rules.
The submarine returned to Bolshoi Kamen, a military shipyard and a navy base near Vladivostok. Russian television stations broadcast the footage of the submarine sailing toward the harbor.
Dygalo said the submarine had 208 people aboard, including 81 servicemen, and was to be commissioned by the navy later this year.
Russian news agencies quoted officials at the Amur Shipbuilding Factory as 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 heading 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.