CLAIRTON, Pa. (AP) — After an explosion rocked a steel plant outside Pittsburgh, workers scrambled into the wreckage alongside firefighters directing streams of water, as black smoke rose into the air.

By the time the scene was secure, two people were dead and more than 10 others were injured, including one who spent hours trapped in rubble left Monday by the blast that was powerful enough to shake nearby homes.

Video from Pittsburgh television station WTAE showed workers in orange jumpsuits and hardhats running toward the wreckage at the U.S. Steel coking plant in Clairton. A reporter described seeing someone being pulled from the debris and loaded onto a gurney. The station’s footage later showed a mountain of charred rubble spilling from a hole left by demolished walls.

Investigators were working Tuesday to determine the cause of the blast, which jolted a region of the state synonymous with steel for more than a century. 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 fatal accidents 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?”

Investigating the blast's cause

Company officials were expected to join Gov. Josh Shapiro at a news conference scheduled for Tuesday afternoon.

At a news conference Monday, Scott Buckiso, U.S. Steel’s chief manufacturing officer, gave no details about the damage or casualties. He said the company, now a subsidiary of Japan-based Nippon Steel Corp., was working with authorities.

The county medical examiner's office identified one of the dead as Timothy Quinn, 39.

The Allegheny County Police Department said five people were hospitalized in critical but stable condition Monday night, and five others had been treated and released. Other individuals were treated for injuries at the scene, but the department said it did not have an exact number.

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

In a statement, the United Steelworkers union, 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, called for an independent investigation and a re-evaluation "as to whether the Clairton plant is fit to keep operating.”

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.

Air quality concerns

The plant, a massive industrial facility along the Monongahela River, converts coal to coke, a key component in the steelmaking process. It is considered the largest coking operation in North America and is one of four major U.S. Steel plants in Pennsylvania.

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 — 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. The agency said its monitors did not detect levels of soot or sulfur dioxide above federal standards.

The US Steel buyout

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 and 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” that gives the U.S. government a say in some matters. The deal came 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 of 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.