Three researchers suffocated to death at a plant of South Korea's Hyundai Motor, Yonhap news agency said Tuesday.
The researchers were found by another employee lying in the vehicle performance test chamber of the plant in the country's southeastern port city of Ulsan at about 3:00 p.m. local time (0600 GMT). They were taken to hospital but all died.
They were believed to have suffocated while working in the chamber, about the size of one vehicle, to test a driving performance.
Exhaust emissions were estimated not to be discharged to the outside during the testing in the chamber.
Police were investigating the exact cause of the accident.