Ministry of Employment and Labor car enters the factory for the joint investigation in Ulsan, Korea, Nov. 20. Yonhap

Police and forensic agency authorities plan to conduct a joint investigation Wednesday into the cause of an accident that left three people dead at a Hyundai Motor plant in the southeastern city of Ulsan.

On Tuesday, three researchers — two Hyundai employees and one affiliated with a subcontractor — were found unconscious at a test chamber where they were conducting a car performance test. They were taken to nearby hospitals but were pronounced dead.

The Ulsan Metropolitan Police said a joint on-site inspection of the test chamber will take place together with the National Forensic Service, the labor ministry and the Korea Occupational Safety and Health Agency to look into safety facilities and details of the performance test.

The victims are presumed to have been suffocated by toxic gas in the enclosed space while performing test drives and engine idling tests.

Police have requested autopsies on the bodies to determine the cause of death. (Yonhap)