CLAIRTON, Pa. (AP) — Explosions at a U.S. Steel plant near Pittsburgh left one dead, two missing and at least nine others in hospitals Monday, with emergency workers searching the badly charred rubble for victims, officials said.

The explosions sent black smoke spiralling into the midday sky in the Mon 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 and two were currently believed to be unaccounted for. Others were treated for injuries, Reigner said.

Allegheny County Emergency Services said a fire at the plant started around 10:51 a.m. The explosions 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.”

Pennsylvania Gov. Josh Shapiro said via X that “multiple explosions” occurred at the facility.

Allegheny Health Network said it had seven patients from the explosion being treated by its hospitals. It did not provide information about the patients’ conditions. University of Pittsburgh Medical Center said it is treating two patients at UPMC Mercy, the region’s only level one trauma and burn center.

Clairton residents like Amy Sowers, 49, felt an explosion nearby. Sowers, who was sitting on her porch located less than a mile from the plant, felt her house shake.

“I could see smoke from my driveway,” she said. “We heard ambulances and fire trucks from every direction.”

Sowers decided to leave the area after she said she smelled a faint smell in the air. Sowers, who grew up in Clairton, has seen several incidents at the plant over the years. Despite health concerns, Sowers said many residents cannot afford to leave.

A maintenance worker was killed in an explosion at the plant in September 2009. In July 2010, another explosion injured 14 employees and six contractors.

“Lives were lost again,” Sowers said. “How many more lives are going to have to be lost until something happens?”

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.

In a statement, U.S. Steel said an “incident” occurred at the plant’s coke oven batteries 13 and 14. The company, now a subsidiary of Japan-based Nippon Steel Corp., said emergency teams were immediately dispatched to the scene, but it gave no other details about the cause of the explosions, casualties or damage.

The company’s CEO, David Burritt, said in the statement that U.S. Steel is working with authorities to investigate the cause.

The plant converts coal to coke, a key component in the steel-making process. To make coke, coal is baked in special ovens for hours at high temperatures to remove impurities that could otherwise weaken steel. The process creates what’s known as coke gas — made up of a lethal mix of methane, carbon dioxide and carbon monoxide.

Clairton Mayor Richard Lattanzi said his heart goes out to the victims of Monday’s explosions.

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

The Allegheny County Health Department said it is monitoring the explosions 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.

According to the company, the plant 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 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.