An amateur video posted on the news portal Sina showed black smoke billowing over the plant as a group of people, their clothing burned off, huddled by the factory gate awaiting help. Photos on the site showed charred bodies laid out in the back of a truck and survivors sitting on wooden pallets.

Forty people were killed on the spot and the others died “one after another” on the way to the hospital, People’s Daily reported, quoting hospital employees.

As many as 200 people were working at the time of the explosion, news reports said.

Throughout the morning, critically injured workers flooded local hospitals, which were ill equipped to treat burn victims, according an online news service operated by The Oriental Morning Post in Shanghai. At the Kunshan Traditional Chinese Medicine Hospital, relatives knelt on the ground begging for information, the website said. Many severely injured patients were transported to hospitals in Shanghai, some with burns covering 90 percent of their bodies, according to the site.

Calls to the company went unanswered on Saturday.

The accumulation of combustible dust, a byproduct of sanding and polishing aluminum, steel and other metals, is a well-known industrial safety hazard. In 2011, an explosion at a plant in Chengdu that made iPads for Apple killed three workers and injured 15. Investigators blamed the accident on aluminum dust that had collected in ventilation ducts.

According to a Shanghai newspaper, The Xinmin Evening News, some employees at the Zhongrong plant had complained about poor ventilation and the buildup of dust in the polishing workshops. One man told the newspaper that two of his relatives had described the dust as intolerable. The man said that his son-in-law had quit as a result but that a brother-in-law had kept working at the plant. The brother-in-law, he told the newspaper on Saturday, was still missing.