Officials are investigating the cause of a Pennsylvania steel plant explosion that killed 2

CLAIRTON, Pa. -- An explosion rocked a steel plant outside Pittsburgh, leaving two dead and 10 others injured, including a person who was rescued from the smoldering rubble after hours of being trapped.

The explosion sent black smoke spiraling into the midday Monday 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 in Clairton started late Monday morning. Officials said they had not isolated the cause of the blast.

The rumbling from the explosion, and several smaller blasts that followed, jolted the community about 15 miles (24 kilometers) southeast of Pittsburgh. Amy Sowers, who was sitting on her porch 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, 45, grew up in Clairton and has seen several incidents at the plant over the years.

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

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

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.

According to the company, the plant has approximately 1,400 workers.

In a statement, the United Steelworkers, which represents many of the Clairton plant’s workers, said it had representatives on the ground at the plant and would work to ensure there is a thorough 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.”

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

It's not the first explosion at the plant. A maintenance worker was killed in a blast 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 fines, which were later reduced under a settlement agreement.

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 but were not seriously injured.

The plant, a massive industrial facility along the Monongahela River, 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 steelmaking 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.

The county health department initially told residents within 1 mile (1.6 kilometers) of the plant to remain indoors and close all windows and doors, but lifted the advisory later Monday. It said its monitors didn't detect levels of soot or sulfur dioxide above federal standards.

U.S. Steel has been a symbol of industrialization since it was founded in 1901 by J.P. Morgan, Andrew Carnegie and others. It’s been the icon of the American steel industry that once dominated the world market until Japan, then China, became preeminent steelmakers over the past 40 years.

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, Beatrice Dupuy in New York City and Audrey McAvoy in Honolulu contributed to this report.