One worker was killed and five others were injured in a fire at a water recycling center in Goyang, northwest of Seoul, on Friday, local media reported.

Seven workers were removing an old generator no longer in use when a gas explosion occurred, smashing windows and walls of the building, according to YTN television in Seoul.

Some witnesses reported hearing a loud boom or explosion sound before the fire began at around 11:05 a.m. (local time), it said.

Fire officials suspect that the explosion occurred after a spark came into contact with gas leaking from a valve that was accidentally opened while cutting gas pipes to dismantle the generator.

Police are investigating the exact cause of the accident in cooperation with forensic experts, fire officials and gas experts.