By Richard Galpin
BBC News, Moscow
The disaster that befell the K-152 Nerpa on Saturday was one of the worst accidents in a long history of incidents involving Russian submarines.
The Nerpa disaster damages Russia's reputation as an arms supplier
It left some 20 people dead and another 21 injured, and Russian authorities have announced that a day of mourning will be held on Tuesday in the far east of the country.
The investigation continues to focus on why the fire extinguishing system on the nuclear sub was apparently activated without warning while the vessel was undergoing tests in the Sea of Japan before entering service in the navy.
As is typical on Russian submarines, the system uses the gas freon, which suppresses fires quickly by displacing oxygen.
But it can also be lethal for any crew members who are still in the area when the gas is released.
All those who died are thought to have suffocated.
So there are two key questions the investigators now need to answer.
Firstly - why was the fire extinguishing system activated? Was it a technical fault with the system itself or was it triggered by human error?
Secondly - why did it lead to so many casualties? Did all those on board at the time have the right safety equipment (emergency breathing apparatus) and if they did, had they been trained to use it?
As soon as the submarine returned to its base near the port city of Vladivostok on Sunday, investigators went on board to examine it.
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
But they continue to make only the most general of comments about the "irregular" or "unsanctioned" activation of the fire system.
It is possible we will never know the full story as it involves highly sensitive information about the Russian armed forces.
The K-152 Nerpa is an attack submarine. According to experts there are 14 of its kind currently in service.
"I would assume that these submarines form the backbone of the nuclear attack capability of the Russian navy," says military analyst Pavel Felgenhauer.
"They are considered to be the quietest of all [Russia's] submarines and are therefore very important as they can launch long-range cruise missiles."
But in the case of the K-152 Nerpa, many experts believe it was an accident just waiting to happen.
Construction work which began back in the early 1990s is believed to have soon ground to a halt in the turmoil which followed the collapse of the Soviet Union.
"In 1998 they told me it was 75% complete and the reactor had been activated," says the defence expert Alexander Goltz.
But it was to be another 10 years before it was finally launched into the Sea of Japan for testing.
"They would claim this submarine is very modern, but all the systems are very old," says Mr Goltz.
"Do we know when they installed the fire system and what has happened to it over the years?"
Years of neglect
Experts say that a lot of Russian military production has relied on stockpiles from the Soviet era which are now dwindling.
And if the parts used in the construction of military equipment are old and in short supply, so too are the highly-skilled technicians and engineers needed to assemble them.
With very few submarines produced over the past two decades, it has been difficult for specialists to either maintain or develop their skills - and that is assuming they have not already retired or left the country.
"The test [of the K-152 Nerpa] was the first such test since 1995," says Alexander Goltz.
As the Nerpa sailed through the Sea of Japan last Saturday, there were more than 200 people on board.
Most were civilian technicians from the shipyard where the submarine was built. It is thought they were instructing the navy crew on how to operate the submarine.
There are many theories about what triggered the fire-extinguishing system.
It is telling that some experts even believe it might have been someone lighting a cigarette.
If the full truth about the accident is ever revealed it will be too late to prevent the damage to Russia's reputation as an arms supplier.
The main Indian news agency PTI, has confirmed that Delhi was planning to lease the Nerpa and a second nuclear submarine of the same class from next year.
India is very keen to get hold of the submarines so it can push ahead with plans to develop a sea-launched nuclear weapons system.
But those plans have now been delayed, at the very least.