The Vietnamese government Thursday ordered Taiwan’s Formosa Plastics Group to pay a $500 million fine for discharging toxic waste from its steel plant in the country — the highest ever fine imposed on a single company in Vietnam. Formosa Ha Tinh Steel on Tuesday admitted responsibility for the disaster that caused massive fish deaths in coastal provinces of Vietnam in April, Reuters reported head of the Government Office Mai Tien Dung saying.

The spill sparked public outrage across Vietnam and three successive weekends of protests, with demonstrators venting their fury at both Formosa and the government, accusing them of a cover-up.

“Violations in the construction and testing operations of the plant are the causes for serious environment pollution killing a massive amount of fish,” Dung said at a news conference, adding: “Formosa has admitted responsibility for the fish deaths in four central provinces and committed to publicly apologize for causing severe environmental incidents.”

Formosa operates a newly set up plant set to become the biggest of its kind in Southeast Asia — one of the single biggest investments by a foreign firm in Vietnam.

According to investigation results released Thursday by Vietnam’s Natural Resources and Environment Ministry, the toxic waste leaked from underground pipes laid at the plant in April, the Nikkei Asian Review reported. The material later reached the sea off the coast of Ha Tinh, killing a large number of fish, the report said. Since early April, about 70 tons of sea fish in the provinces of Ha Tinh, Quang Binh, Quang Tri and Thua Thien-Hue have died, local online newspaper VN Express reported.

Formosa Plastics implemented countermeasures, such as moving the pipes in question to the surface to make it easier to check for leaks.

A video clip played at Thursday’s news conference showed Tran Nguyen Thanh, the chairman of Formosa Ha Tinh Steel, expressing regret over the incident. “We deeply hope the Vietnam people can forgive us,” he said.