Deaths in Russian Sub Accident

It appears that a fire suppression system activated, and people suffocated before they could don emergency breathing gear.

This may come down to training, as the people on board may not have know how and when to use breathing gear.

Technical error in article: Freon (more likely a similar CFC like halon) is described as a toxic gas. It isn't, but in high enough concentrations, it can suffocate you, much like a pure Nitrogen atmosphere did at a NASA installation a few years back.