GOYANG (Yonhap News) ― Four South Korean mechanics died of apparent asphyxiation caused by refrigerant gas in an underground machine room at a major retail outlet near Seoul, officials said Saturday.

The four were working to fix a refrigeration system in the machine room in E-Mart in Ilsan, northwest of Seoul, until 2:40 a.m., an E-Mart official said, adding that the ventilation system was working and the door was open.

But they were found lying unconscious around 4 a.m., officials said.

Emergency officials tried to resuscitate the mechanics while rushing them to nearby hospitals where they were later pronounced dead.

“We presume they died of asphyxiation at the machine room,” said Kim Kil-soo, an emergency official handling the case.

The gas is widely used as a refrigerant, but it contains chlorine, a toxic chemical, Kim said.

He also said the four were not wearing any respiratory protection when they were found.

Forensic experts plan to conduct an autopsy on the bodies to determine the exact cause of death, police official Yoon Hee-seog said. E-Mart is South Korea’s second-largest retail outlet and has chains across the country.