Various media outlets are reporting that the Russian Navy has suffered another, fatal submarine accident.

A Russian naval spokesman tells Reuters that 21 sailors died in the mishap, and 21 more were injured. The name of the vessel and the accident location were not disclosed, but Russian sources indicate that the destroyer Admiral Tributs was assisting the rescue operations. Normally based at Vladivostok, the largest base in the Russian Pacific Fleet, the Tributs participation suggests that that mishap occurred near the naval facility.

We also know that the unnamed sub had a crew that was much larger than normal. According to Russian sources, at least 208 personnel were on board the sub at the time of the mishap. A “standard” crew for a Russian Akula-class attack submarine is between 50-100 men; a Los Angeles-class attack boat of the U.S. Navy has a complement of 129.

Why the over-sized crew? Galrahn at InformationDissemination offers the most logical explanation. The sub that suffered the fatal mishap is (likely) the same attack boat that Russia has leased to the Indian Navy. Delhi has signed a 10-year, $650 million deal to lease the vessel from Russia, with delivery expected next year.

Citing Russian press accounts, Galrahn reported late last month that the Akula-type sub was undergoing sea trails off the Soviet Pacific coast–the same area where the accident occurred. The expanded crew included Russian and Indian sailors, as well as shipyard personnel. At this point, it’s unclear if Indian personnel and Russian contractors were among the dead and injured.

Initial reports suggest the mishap began with the “unsanctioned functioning of the fire extinguishing systems.” The Russian spokesman didn’t explain what that means, but it suggests that crew members were caught in spaces where oxygen was in short supply; without access to emergency breathing equipment–and training in how to use it–survival would be difficult.

The accident was the latest in a long string of submarine tragedies in the Soviet and Russian navies. Eight years ago, an Oscar II attack submarine, designated the Kursk, sank in the Barents Sea after an accident. The entire crew–more than 100 men–perished. In 1989, 42 of the 69 crew members of a “Mike” class boat were lost after their submarine sank off Norway. That accident was blamed on a fire that caused mechanical malfunctions on board the sub.

During the “Mike” disaster, most of the crew made it out of the stricken boat, but there weren’t enough life rafts, and many of the crew members perished in the frigid waters of the North Atlantic. With more than 200 men crowded on that Akula, it’s not difficult to envision a scenario where men couldn’t access emergency breathing equipment, or were not trained on its proper use.

By some estimates, more than 500 Russian sailors have died in submarine accidents over the last 50 years. The sub involved in the latest accident is believed to be a fairly new hull that was overhauled in anticipation of the transfer to India.