FILE - Part of the U.S. Steel Clairton Coke Works is seen May 2, 2019, in Clairton, Pa. (AP Photo/Gene J. Puskar, File)

An explosion at a U.S. Steel plant near Pittsburgh left one dead and dozens injured or trapped under the rubble Monday, with emergency workers on site trying to rescue victims, officials said.

The explosion sent black smoke spiralling into the midday sky in the Monongahela Valley, a region of the state synonymous with steel for more than a century. An Allegheny County emergency services spokesperson, Kasey Reigner, said one person died in the explosion and two were currently believed to be unaccounted for. Multiple other people were treated for injuries, Reigner said.

Allegheny County Emergency Services said a fire at the plant started around 10:51 a.m. The explosion sent a shock through the community and led to officials asking residents to stay away from the scene so emergency workers could respond.

“It felt like thunder,” Zachary Buday, a construction worker near the scene, told WTAE-TV. “Shook the scaffold, shook my chest, and shook the building, and then when we saw the dark smoke coming up from the steel mill and put two and two together, and it’s like something bad happened.”

Dozens were injured and the county was sending 15 ambulances, on top of the ambulances supplied by local emergency response agencies, Reigner said.

Air quality concerns and health warnings

The plant, a massive industrial facility along the Monongahela River south of Pittsburgh, is considered the largest coking operation in North America and is one of four major U.S. Steel plants in Pennsylvania that employ several thousand workers.

Democratic Sen. John Fetterman, who formerly served as the mayor of nearby Braddock, called the explosion “absolutely tragic” and vowed to support steelworkers in the aftermath.

“I grieve for these families,” Fetterman said. “I stand with the steelworkers.”

The Allegheny County Health Department said it is monitoring the explosion and advised residents within 1 mile (1.6 kilometers) of the plant to remain indoors, close all windows and doors, set air conditioning systems to recirculate, and avoid drawing in outside air, such as using exhaust fans. It said its monitors have not detected levels of soot or sulfur dioxide above federal standards.

The plant converts coal to coke, a key component in the steel-making process. According to the company, it produces 4.3 million tons (3.9 million metric tons) of coke annually and has approximately 1,400 workers.

The plant has a long history of pollution concerns

In recent years, the Clairton plant has been dogged by concerns about pollution. In 2019, it agreed to settle a 2017 lawsuit for $8.5 million. Under the settlement, the company agreed to spend $6.5 million to reduce soot emissions and noxious odors from the Clairton coke-making facility.

The company also faced other lawsuits over pollution from the Clairton facility, including ones accusing the company of violating clean air laws after a 2018 fire damaged the facility’s sulfur pollution controls.

In 2018, a Christmas Eve fire at the Clairton coke works plant caused $40 million in damage. The fire damaged pollution control equipment and led to repeated releases of sulfur dioxide, according to a lawsuit. Sulfur dioxide is a colorless, pungent byproduct of fossil fuel combustion that can make it hard to breathe. In the wake of the fire, Allegheny County warned residents to limit outdoor activities, with residents saying for weeks afterward that the air felt acidic, smelled like rotten eggs and was hard to breathe.

In February, a problem with a battery at the plant led to a “buildup of combustible material” that ignited, causing an audible “boom,” the Allegheny County Health Department said. Two workers who got material in their eyes received first aid treatment at a local hospital but were not seriously injured.

Last year, the company agreed to spend $19.5 million in equipment upgrades and $5 million on local clean air efforts and programs as part of settling a federal lawsuit filed by Clean Air Council and PennEnvironment and the Allegheny County Health Department.

The fire at the Clairton plant knocked out pollution controls at its Mon Valley plants, but U.S. Steel continued to run them anyway, environmental groups said.

The lawsuits accused the steel producer of more than 12,000 violations of its air pollution permits.

Environmental group calls for an investigation

David Masur, executive director of PennEnvironment, another environmental group that has sued U.S. Steel over pollution, said there needed to be “a full, independent investigation into the causes of this latest catastrophe and a re-evaluation as to whether the Clairton plant is fit to keep operating.”

In June, U.S. Steel and Nippon Steel announced they had finalized a “historic partnership,” a deal that gives the U.S. government a say in some matters and comes a year and a half after the Japanese company first proposed its nearly $15 billion buyout of the iconic American steelmaker.

The pursuit by Nippon Steel for the Pittsburgh-based company was buffeted by national security concerns and presidential politics in a premier battleground state, dragging out the transaction for more than a year after U.S. Steel shareholders approved it.

Clairton Mayor Richard Lattanzi said his heart goes out to the victims of Monday explosion.

“The mill is such a big part of Clairton,” he said. “It’s just a sad day for Clairton.”

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Levy reported from Harrisburg, Pennsylvania; Casey reported from Boston and Whittle reported from Portland, Maine. Associated Press reporters Holly Ramer in Concord, New Hampshire, and Beatrice Dupuy in New York City contributed to this report.