Yet 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.
Overcrowding may have been a significant factor on Saturday.
The submarine being tested had 208 people aboard, including 81 seamen, according to Russian navy spokesman Capt. Igor Dygalo. Yet Russian news agencies said a sub of this type normally carries only a crew of 73.
"A submarine is the most vulnerable during trials. With both navy and civilian personnel on board, it's very dificult to keep such a large number of people organized," Gennady Illarionov, a retired submarine officer, told the RIA Novosti news agency.
The victims suffocated after the submarine's fire-extinguishing system released Freon gas, said Vladimir Markin, an official with Russia's top investigative agency. He said forensic tests found Freon in the victims' lungs.
Seventeen civilians and three seamen died in the accident and 21 others were hospitalized after being evacuated to shore, Dygalo said, adding that none of the injuries were life-threatening.
"The submarine's nuclear reactor was operating normally and radiation levels were normal," Dygalo said, explaining that the accident affected two sections of the submarine closest to the bow.
Markin's agency, the Investigative Committee under the Prosecutor General's office, has launched a probe into the accident, which he said will focus on what activated the firefighting system and possible violations of submarine operating rules.
Lev Fyodorov, a top Russian chemical expert, agreed that the Freon pushed oxygen out, causing those inside to die of suffocation. But he wondered why the individual breathing kits that everyone on board is supposed to have did not keep people from dying.
"People on board the sub may have failed to use their breathing equipment when they found themselves in an emergency," he told the AP.
Igor Kurdin, a retired navy officer who heads an association of former submariners, told Ekho Moskvy radio that the high death toll probably resulted from shipyard workers who lacked experience in dealing with the breathing kits.
A siren warning the crew that the firefighting system was turning on also may have failed, RIA Novosti quoted an unidentified navy official as saying, so those on board might not have realized that Freon was being released until it was too late.
The submarine returned Sunday to Bolshoi Kamen, a military shipyard and a navy base near Vladivostok. Officials at the Amur Shipbuilding Factory said they built the submarine and it is called the Nerpa. Dygalo said it was to be commissioned by the navy later this year.
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.
The U.S.-based intelligence risk assessment agency Stratfor said the Akula is an established design, with the Nerpa being the 11th ship of the class.
"Such a catastrophic accident calls into question the way the Russian navy has sustained its institutional knowledge in terms of design expertise, not to mention issues of quality control, both in fabrication and inspection," Stratfor 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.
Illarionov said the accident appeared to reflect the loss of crucial skills in conducting sea trials.
"During the Soviet times, we commissioned three to five submarines a year, and now we get just one in five years," Illarionov was quoted by RIA Novosti as saying. "People forgot caution and lost their skills."
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.
