The K-152 Nerpa was carrying many more people than usual
An inquiry is under way into Saturday's gas poisoning on a Russian nuclear submarine in the Pacific that left 20 people dead, including 17 civilians.
Another 21 people were left ill in what officials believe was an "unsanctioned" activation of an automatic firefighting system that released freon gas.
Experts speculated that the presence of many civilians aboard during sea trials may have elevated the death toll.
But this has not been confirmed by officials investigating the accident.
The accident on the Nerpa, or Akula-class, attack vessel occurred during trials in the Sea of Japan.
Three seamen were among those killed in the accident, which happened in the nose of the submarine, officials said.
But the nuclear reactor, which is in the stern, was not affected and there was no radiation leak, Russian Pacific Fleet spokesman Igor Dygalo said.
Personnel working in front section of submarine affected when fire extinguishers set off unexpectedly
Affected area may have been sealed off, trapping personnel inside. There were more staff than usual on board as submarine was undergoing tests
Fire extinguishers release freon gas, which displaces breathable oxygen, causing workers to suffocate
Twenty people die and 21 are injured
The submarine is due to be leased to India, and Indian naval personnel were due to travel to Vladivostok earlier this month to train on board the submarine ahead of its transfer, according to the website Indian Defence.
Preliminary results suggested that the "unsanctioned activation" of the firefighting system caused the tragedy, Russian investigators said on Monday.
RUSSIAN SUBMARINE DISASTERS
6 Sept 2006: Two die after fire in Viktor-III class Daniil Moskovsky
28 Aug 2003: Nine die after decommissioned November class K-159 sinks
12 Aug 2000: 118 die in sinking of Oscar-II class Kursk
7 Apr 1989: 42 die after fire in Soviet-era Komsomolets
However, they said that it was still unclear what triggered the system. The submarine had 208 people aboard, 81 of whom were servicemen.
However, a vessel of this type usually carries only 73 people, Russian expert Ruslan Pukhov was quoted as saying by the Ekho Moskvy radio.
Several former Russian mariners have suggested that overcrowding and inexperience of the civilian personnel - engineers and shipyard workers - in dealing with the breathing apparatus may have elevated the death toll.
"I cannot exclude that among those civilians who found themselves on board, not everyone had the equipment and that those who did may not have known how to use it," former navy captain Gennady Illarionov was quoted as saying by Russia's Ria Novosti news agency.
Some reports in the Russian media also say that a siren warning may have failed when the firefighting system activated.
Freon gas displaces oxygen from the air to put out the fire. But in this case it appears such large quantities were released that anyone still trapped inside the area faced suffocation.
Russia's worst submarine disaster happened in August 2000, when the nuclear-powered Kursk sank in the Barents Sea. All 118 people on board died.
The then president, Vladimir Putin, was criticised for being slow to react to the incident and reluctant to call in foreign assistance.