Police investigating a deadly gas leak at a local power plant found on Wednesday that employees did not comply with safety rules before the accident which left one dead and three others in critical condition.

The incident occurred at Hyundai Greenpower in Dangin, South Chungcheong Province, on Tuesday evening.

Police said six out of the nine workers on the scene didn’t wear gas detectors. One worker was poisoned by the gas while doing maintenance work on the plant’s boiler. His co-worker, who came to his rescue, also inhaled the toxic gas, officials said.

The boiler uses blast furnace gas, a type of gas that is a byproduct of iron ore smelting, and produces toxic gases, including carbon dioxide, carbon monoxide and nitrogen.

“It looks like the blast furnace gas leaked while workers were on maintenance jobs,” an official said. “We will investigate whether they were following the safety rules properly.”

Investigators said they are looking into whether the company conducted safety checks and what have caused the gas leak.

Labor Minister Phang Ha-nam apologized and pledged tougher inspections a day after the accident.

“I feel like I need to apologize to the public,” the minister said at a meeting of the National Assembly Environment and Labor Committee.

“I feel ashamed to see this kind of accident happening again even though we took preventive measures.”

He added that the government will launch intensive safety inspections of every Hyundai Steel Co. facility and will consider taking legal action.

A similar accident at Hyundai Steel Co., the nation’s second-largest steelmaker, six months ago killed five people, sparking safety concerns at industrial sites. Hyundai Greenpower is a partner of the steel firm.

Five workers suffocated to death in May after argon gas leaked from the furnace.

The victims were removing the scaffolding that had been used for regularly scheduled repair work from inside the furnace when the argon gas leaked from a valve, leading to a lack of oxygen in the container.

The prosecution indicted 14 officials at Hyundai Steel on charges of accidental homicide while the labor ministry ordered the steelmaker to correct a total of 1,123 violations of the industrial safety act.

By Cho Chung-un (christory@heraldcorp.com)