[SunkenCiv]

Among the last things the sub crew reported was a fire — it isn’t unlikely that the fire asphyxiated everyone on board shortly thereafter, regardless of what other problems had arisen, and aboard submarines, one problem generally proliferates into multiple problems.

[Calvin Locke]

The Soviets ended up losing a sub that had a fire. The fire compromised the auxiliary air system, so when the fire crew plugged their respirators into it, they dropped dead.

Most of the crew got off on the surface, before it sank. I think only one of the officers in the escape pod managed to escape before it too, sank.