One worker was pulled from the wreckage hours after the blasts at Clairton Coke Works sent black smoke spiralling into the sky. Authorities later said a second person had died.
Firefighters battled flames and heavy smoke that billowed out of the plant, which is owned by US Steel, a subsidiary of Nippon Steel.
Allegheny Health Network said it treated seven patients from the plant, and discharged five within a few hours. University of Pittsburgh Medical Centre said it is treating three patients at UPMC Mercy, the region's only level one trauma and burns centre.
The explosions 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.Â
"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."
Allegheny County Police Assistant Superintendent Victor Joseph said the probe into the explosions would be "a time-consuming technical investigation."
David Burritt, president and chief executive officer of US Steel, said in a statement the company was working with local authorities to discover the cause.
Pennsylvania Governor Josh Shapiro confirmed on X there were multiple explosions and that his administration was in touch with local officials.
Clairton Mayor Rich Lattanzi said it was a horrible day for the city, about 32km south of Pittsburgh, long known as America's Steel City.
"The mill is such a big part of Clairton," he said. "It's just a sad day for Clairton."
US Steel has produced steel in the area since the late 19th century, but in recent decades, the industry has been in decline, leading to plant closures and restructurings.
with AP
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