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

There are no confirmed fatalities at the Clairton Coke Works, said Abigail Gardner, director of communications for Allegheny County. The explosion sent black smoke spiralling into the midday sky in the Monongahela Valley, a region synonymous with the state for more than a century.

The Allegheny County Emergency Services said a fire at the plant started around 10:51 a.m. and that it has transported five people. The agency did not provide any more details on those people transported and would only say it was an “active scene.”

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.”

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.

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.

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.

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

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