Staff at Bum Han Industrial Company were doing maintenance work at a steel fabricator of Hung Nghiep Formosa Ha Tinh Steel Company in Ky Anh Town at around 3 p.m. Thursday when something happened that might have caused a carbon dioxide leakage at the site, authorities said.

The accident killed two workers on the spot, and one died in the hospital, authorities added.

Phan Thi Xuan Lieu, director of Ky Anh General Hospital, said the three workers died due to disrupted circulation.

"The company, upon bringing the three workers to the hospital, said they died due to asphyxiation caused by carbon dioxide... multiple steps would be needed to verify this fact," she said.

Ha Tinh authorities have withheld the identities of the three deceased so as not to impede the investigation.

The Bum Han company, based in Ho Chi Minh City, constructs pipes for the Formosa plant. Dozens of company workers now work at the steel producer.

Taiwanese company Formosa hit the headlines in 2016 for causing one of the biggest environmental disasters in Vietnamese history, polluting 200 kilometers (125 miles) of coastline in Ha Tinh and three nearby provinces.

The company’s test-run led to the discharge of toxic substances into the sea, including phenol, cyanide and iron hydroxide, which devastated sea life and local economies dependent on fishing and tourism.

The environment ministry said affected regions might need a decade to completely recover from the incident, while experts predicted the disaster may set Vietnam’s economy back by years.