Three Chinese fishermen died on Thursday when a fire started on their boat inside the western South Korean territorial waters, officials reported.

On Thursday morning around 10:00 AM, the boat was reportedly sailing in waters near Hong Island, by the southwestern city of Mokpo. A South Korean coast guard attempted to board the boat for inspection.

According to Yonhap report, South Korean coast guard broke the window to the cockpit and engine room after discovering both were locked. They proceeded to throw in three flash grenades. A fire soon broke out, which the officers reportedly put out with a fire extinguisher. They then came upon three unconscious crewmembers in the engine room. 

The three bodies were moved to a patrol boat for emergency treatment, but were soon pronounced dead. Coast guard officials are unsure of the exact cause of the fire but speculate that the three are believed to have died from inhaling too much smoke. The fishing boat and three bodies are expected to arrive at Mokpo Friday morning, where the Korean coast guard will continue with an investigation.