Aboard a submarine conducting tests, a fire protection device was accidentally triggered around 6 pm, releasing a chloro-fluoro gas. The refrigerant did not reach the nuclear part of the vessel, yet still spread quickly into two of the six submarine compartments: 17 civilians and 3 marines fatally succumbed, and 21 others were intoxicated to varying degrees. According to authorities, a defect in the fire protection system would have caused the accident.

Some crew members were sleeping at the time. The consequences of this accident were also exacerbated due to the presence of 208 people (including 120 naval engineers and workmen) aboard a vessel normally designed to carry 73 officers; a former marine noted that the lack of experience among civilians on board in the use of oxygen masks might explain the high number of casualties.