The high casualty count was attributed in part to the large number of civilians on board who were assisting with the testing before commissioning.

Three of the dead were Russian naval personnel and the rest were civilian employees of the Vostok, Zvezda, Era, and Amur shipbuilding yards.

[3] For this reason, it had a much larger than usual complement aboard, totaling 208 people, 81 military personnel and 127 civilian engineers from the shipyards responsible for building and outfitting the submarine.

[3] The submarine's fire extinguishing system was triggered, sealing two forward compartments and filling them with R-114B2 gas (dibromotetrafluoroethane, known as khladon in Russian).

[6] The gas, a haloalkane refrigerant, is used in the Russian Navy's LOKh (lodochnaya obyemnaya khimischeskaya – "submarine volumetric chemical") fire-suppressant system.

In high concentrations, it can cause narcosis, which progresses by stages into excitation, mental confusion, lethargy, and ultimately asphyxiation.

[9] Following the incident, the Udaloy-class destroyer Admiral Tributs and the rescue vessel Sayany were dispatched from Vladivostok to provide assistance to the stricken submarine.

[11] On 10 November 2008, a Russian Navy statement blamed the disaster on an "unsanctioned operation" of the fire suppression system aboard Nerpa.

[4] Three days later, naval investigators announced that a crewman, named unofficially as Dmitry Grobov, had turned on the system "without permission or any particular grounds".

He noted that civilian observers would be untrained in the proper response to the release of the boat's firefighting gas, which would be preceded by a specific light and sound signal, after which all on board are supposed to put on oxygen masks to allow them to survive during the 30-minute period required to ventilate the compartments before they can be reopened.

Mikhail Barabanov, editor-in-chief of Moscow Defense Brief, said the potential for accidental discharge of the fire-suppression system on Russian submarines is not unheard of, but this type of incident does not normally result in fatalities.

His estimation of the events that compounded the loss of life were: "the fire alarm failed to work, so passengers did not realize that the gas started to displace oxygen in the affected compartments.

[15] Russian President Dmitry Medvedev ordered Defence Minister Anatoliy Serdyukov to undertake an immediate investigation into the causes of the accident.

[4] Colonel General Aleksandr Kolmakov, the First Deputy Minister of Defence, and Admiral Vladimir Vysotsky, the Navy Commander-in-Chief, were sent to the Far East region to oversee the investigation.