Multiple explosions at a U.S. Steel plant near Pittsburgh killed at least one person and injured 10 others, authorities said on Monday, and emergency crews were searching through the rubble for an employee believed to be missing.

The blasts at the Clairton Coke Works — part of a sprawling industrial complex along the Monongahela River — took place just before 11 a.m. ET. Firefighters battled flames and heavy smoke that billowed out of the plant, which is owned by U.S. Steel, a subsidiary of Nippon Steel.

Initially, two people were reported missing. One person was found and transported to a local hospital, said Allegheny County Police Assistant Superintendent Victor Joseph at an afternoon briefing. The other individual remains missing, he said.

There was no word yet on a possible cause of the explosion.

“It’s still a rescue mission,” Joseph said, adding that the probe into the explosion would be “a time-consuming technical investigation.”

David Burritt, president and chief executive officer of U.S. Steel, said in a statement that the company was working with local authorities to discover the cause.

Authorities believe they know the general location of the missing employee but were not completely sure, said Scott Buckiso, executive vice president and chief manufacturing officer for U.S. Steel, at Monday’s briefing.

Pennsylvania Gov. Josh Shapiro posted on X that there were multiple explosions at the plant and that his administration was in touch with local officials.

“The scene is still active, and folks nearby should follow the direction of local authorities,” he wrote.

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(Reporting by Rich McKay in Atlanta and Julia Harte in New York; Additional reporting by Ryan P. Jones in Toronto; Editing by Frank McGurty and Lisa Shumaker)