The sailors died from smoke inhalation after a blaze broke out Monday aboard the deep-water vessel, which was conducting a planned exploration of the seafloor in Russian waters, Defense Minister Sergei Shoigu told President Vladimir Putin at a meeting in Moscow. The vessel is linked to a secret nuclear submarine project known as Losharik, RBC news website reported earlier, citing a person it didn’t identify.

The submersible is now at the Northern Fleet’s Severomorsk base, on the Barents Sea coast, and “measures are being taken to examine it and find the causes of the incident,” Shoigu said.

Putin ordered Shoigu to visit Severomorsk to “personally hear all reports” on what led to the fire and report back to him.

“It’s an unusual vessel, we know, it’s a research vessel, the crew was highly professional,” Putin said. Two of the 14 killed were Heroes of Russia and seven were first-rank captains, according to Putin. “This is a great loss for the fleet, for the army as a whole.”

The incident is the worst since 20 people died on a Nerpa nuclear submarine in 2008. The Losharik submarine can operate at a depth of 6,000 meters, according to RBC. The submarine has a titanium housing and had been used for research on the continental shelf, Izvestia newspaper reported in 2012.