PALU, Indonesia (AP) — The death toll following the explosion of a smelting furnace at a Chinese-owned nickel plant on Indonesia’s Sulawesi island rose to 18 on Tuesday, as police ordered the plant to stop operations until an investigation into the incident is completed.

The accident, which occurred on Sunday, was the latest in a series of deadly incidents at nickel smelting plants in Indonesia that are part of China’s ambitious transnational development program known as the Belt and Road Initiative.

Nickel is a key component in global battery production for electric vehicles.

Four Chinese and nine Indonesian workers died instantly on Sunday when the furnace exploded while they were repairing it, said Central Sulawesi police chief Agus Nugroho. Three more victims died a day later while being treated at a local hospital.

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Two more workers died on Tuesday at the hospital, bringing the total number of fatalities to 18, including eight workers from China, said Deddy Kurniawan, a spokesperson for PT Indonesia Morowali Industrial Park, known as PT IMIP, the parent company of PT Indonesia Tsingshan Stainless Steel, where the accident occurred.

The plant is in the Bahodopi neighborhood of Morowali regency.

“We have ordered PT Indonesia Tsingshan Stainless Steel to stop its operation until our entire investigation is completed,” said Nugroho, the police chief, adding that authorities had set up a team to determine whether negligence by the company led to the deaths.

The blast was so powerful it demolished the furnace and damaged parts of the side walls of the building, Nugroho said.

PT IMIP said in a statement on Sunday that the furnace was under maintenance and not operating at the time of the accident. However, “residual slag in the furnace” came in contact “with flammable items,” causing the furnace walls to collapse and the remaining steel slag to flow out.

Rescuers extinguished the fire and evacuated workers after a nearly four-hour operation, Kurniawan said.

About 44 workers are still being treated at a hospital and the company’s clinic on Tuesday with serious to minor injuries, including 11 Chinese nationals, Kurniawan said.

In a news briefing on Monday, Chinese Foreign Affairs Ministry spokesperson Mao Ning expressed condolences for the victims and said that China is “saddened by the casualties caused by the accident.”

She said her ministry is working closely with authorities in Indonesia and has instructed the Chinese Embassy in Jakarta to assist in the aftermath, including ensuring medical treatment is provided to the injured and helping to determine the cause of the accident.

It was the third deadly incident this year at Chinese-owned nickel smelting plants in Central Sulawesi province, which has the largest nickel reserves in Indonesia.

Two dump truck operators were killed when they were engulfed by a wall of black sludge-like material following the collapse of a nickel waste disposal site in April.

In January, two workers, including a Chinese national, were killed in riots that involved workers of the two nations at an Indonesia-China joint venture in neighboring North Morowali regency.

Last year, a loader truck ran over and killed a Chinese worker while he was repairing a road in PT IMIP’s mining area, and an Indonesian man burned to death when a furnace in the company’s factory exploded.

Nearly 50% of PT IMIP’s shares are owned by a Chinese holding company, and the rest are owned by two Indonesian companies. It began smelter operations in 2013 and is now the largest nickel-based industrial area in Indonesia.

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Associated Press writers Niniek Karmini in Jakarta, Indonesia, and Emily Wang Fujiyama in Beijing, contributed to this report.