CLAIRTON, Pa. — An explosion at a U.S. Steel plant near Pittsburgh left two dead and sent at least 10 to hospitals Monday and heavily damaged the sprawling facility, officials said.

One worker was found alive in the wreckage hours after the explosion sent black smoke spiraling into the midday sky in the Mon Valley, a region of the state synonymous with steel for more than a century. Allegheny County Emergency Services said a fire at the plant started around 10:51 a.m.

The explosion, followed by several smaller blasts, could be felt in the nearby community and prompted county officials to warn 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.”

Cause under investigation

At a news conference, Scott Buckiso, U.S. Steel’s chief manufacturing officer, did not give details about the damage or casualties, and said they were still trying to determine what happened. U.S. Steel employees “did a great job” of going in and rescuing workers, shutting down gases and making sure the site was stable.

Buckiso said the company, now a subsidiary of Japan-based Nippon Steel Corp., is working with authorities

U.S. Steel CEO David B. Burritt said the company would thoroughly investigate the cause.

“I end every meeting and every message with the words, ‘Let’s get back to work safely.’ That commitment has never been more important, and we will honor it,” he said in a statement.

Allegheny Health Network said it treated seven patients from the plant, and discharged five within a few hours. University of Pittsburgh Medical Center said it is treating three patients at UPMC Mercy, the region’s only level one trauma and burn center.

Clairton resident Amy Sowers was sitting on her porch, located less than a mile from the plant, and felt her house shake from the blast.

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

Sowers, 49, 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. According to online OSHA records of workplace fatalities, the last death at the plant was in 2014, when a worker was burned and died after falling into a trench.

After the 2010 explosion, the Occupational Safety and Health Administration fined U.S. Steel and a subcontractor $175,000 for safety violations. U.S. Steel appealed its citations and $143,500 in fines, which were later reduced under a settlement agreement.

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

The Clairton coking plant continued to operate after the explosion, although two batteries that were the site of the explosion were shut down, officials 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.

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 explosion.

“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 lifted an advisory it issued earlier in the day telling residents within 1 mile (1.6 kilometers) of the plant to remain indoors and close all windows and doors. It said its monitors have not detected levels of soot or sulfur dioxide above federal standards.

According to the company, the plant 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 an air pollution lawsuit for $8.5 million. Five years later, 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 lawsuit stemmed from a Christmas Eve fire in 2018 that caused $40 million in damage. The fire damaged pollution control equipment and led to repeated releases of sulfur dioxide, according to a lawsuit. 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.

Dr. Deborah Gentile, the medical director of Community Partners in Asthma Care, studied asthma levels after the fire and found twice as many patients sought medical treatment. One of her colleagues found patients living near the plant had increased symptoms of asthma, including coughing, wheezing and shortness of breath.

News of the latest explosion had Gentile questioning how well the facility was being maintained.

“I’m very concerned that they aren’t keeping their equipment up to date and in shape,” she said.

In February, a problem with a battery at the plant led to a “buildup of combustible material” that ignited, causing an audible “boom,” officials said. Two workers received first aid treatment at a local hospital but were not seriously injured.

Environmental group calls for an investigation

David Masur, executive director of PennEnvironment, an 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.

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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.