An autopsy has suggested gas poisoning to be the cause of death after a worker in his 30s lost his life while cleaning a wastewater tank at a Hyundai Steel plant on Tuesday.

According to the Incheon Jungbu Police Station on Wednesday, the National Forensic Service(NFS) delivered such a preliminary opinion with no particular component detected during the autopsy, adding it would have to conduct a thorough analysis to determine the exact cause.

The police, along with the Korea Occupational and Health Agency, planned to inspect protection equipment worn by the workers during cleaning, while looking into whether Hyundai Steel and the outsourcing cleaning company had followed safety regulations.

The labor ministry's regional office has suspended work at the plant as it began a separate probe into possible violations of the Serious Accident Punishment Act and the Occupational Safety and Health Act.

Seven employees, including the 34-year-old deceased worker, collapsed during the cleaning work at Hyundai Steel's Incheon plant Tuesday morning. While six are receiving medical treatment, two are reportedly in critical condition.