A fire aboard a Russian deep-sea research vessel has killed at least 14 personnel, including high-ranking naval officers, all of whom were overcome by toxic fumes, the Defense Ministry said Tuesday in terse statements that deepened the mystery surrounding the incident.

The fire took place Monday, according to various reports, while the crew aboard was conducting topographic surveys of the Arctic sea floor near Severomorsk, the main base for Russia’s Northern nuclear submarine fleet.

The vessel, which is an AS-12 deep sea nuclear-powered submarine nicknamed Losharik, is linked to the Defense Ministry’s unit for undersea intelligence, according to various Russian media, which is tasked with sensitive tasks like mapping and monitoring ocean depths.

The ministry has released nothing on the state of the vessel’s single nuclear reactor. But Norway’s Radiation and Nuclear Safety Authority said there had been no spikes in radiation in the Barents Sea area following the deadly fire, according to Reuters.

President Vladimir Putin, who came under criticism for his slack handling of the Kursk nuclear submarine disaster in 2000, which killed 118 sailors, canceled a scheduled appearance and immediately summoned Defense Minister Sergei Shoigu for a briefing on the blaze.

During a televised meeting, Putin said at least half of those who perished in the Losharik blaze had been of the rank of captain. Two had previously received the Hero of the Russian Federation award, one of the country’s highest honors.

Shoigu praised the sailors. “Fourteen submariners have died of poisoning by fumes from the fire,” he told Putin during the meeting. “The fire was extinguished thanks to the crew’s resolute action.”

Putin ordered Shoigu to Severomorsk to oversee the investigation. The vessel has been recovered from the site of the accident and returned to the base.

Yet much about the sinking still remains a matter of guesswork. It fell to Russia’s RBC online news outlet and the Novaya Gazeta newspaper to identify the vessel as the nuclear-powered AS-12 Losharik.

But it is still unclear how many sailors were aboard the vessel at the time of the accident. While the death toll is known, the secretive craft can accommodate as many as 25 crew.

It is, however, clear that the Losharik is a submersible, and not a submarine. While a submarine operates autonomously, a submersible requires support. According to Barents Observer news portal, the Losharik was carried in the hull of the nuclear powered Podmoskvye, a retooled Delta IV class nuclear submarine, and could reach depths up to 6,000 meters.

Monday’s sinking of the AS-12 marks the worst death toll for an underwater since 2008, when 20 were killed aboard the Nerpa nuclear submarine during a failed test of the vessel’s fire suppression system.

According to Reuters, the Losharik, first launched in 2003, is made out of a series of inter-connected spheres, which are stronger than the conventional submarine construction and allow the vessel to resist water pressure at great depths.

The sinking of the Kursk submarine in 2000, soon after Putin took office, focused official attention on the state of the military and its hardware, which had been subject to underfunding and neglect after the collapse of the Soviet Union.

Since then, Putin has overseen a massive increase in military spending that has allowed the armed forces to renew their equipment and improve training and morale.