Korean automobile manufacturer Hyundai Motor's Ulsan plant is seen in this photo taken in April 8, 2020. Yonhap

Three people died apparently due to suffocation during vehicle testing at a Hyundai Motor plant in the southeastern city of Ulsan, according to police and officials, Tuesday.

The Ulsan Metropolitan Police Agency said three researchers were found unconscious inside a laboratory at Hyundai Motor's Ulsan Plant 4 complex around 3 p.m. They had been conducting a driving test at the lab.

The three individuals, aged in their 40s, 30s, and 20s, were taken to a hospital in critical condition but were pronounced dead later.

According to a company official, the three individuals did not exit the testing chamber after completing the driving test in the laboratory of Plant 4 around 12:50 p.m. They were later found lying on the floor in the laboratory when others went in to check around 3 p.m.

Hyundai Motor's Ulsan plant is subject to the Serious Accidents Punishment Act with more than 10 full-time workers.

The Ulsan Northern Police Station and the Ministry of Employment and Labor are investigating the exact circumstances of the accident.