An explosion at a U.S. Steel plant in Clairton, Pennsylvania, today killed at least one person and injured dozens more, with some still trapped under rubble, authorities said.

Witnesses described feeling a shock wave and then seeing black smoke spew into the sky. Part of a building appeared to have been sheared off, and rubble littered the area.

Two people are missing, the Allegheny County Police Department said in a statement. Local hospitals said at least nine people were being treated for injuries from the explosion.

“We were on lunch, and we felt an explosion,” said Ronnie Fordyce, who said he works at the plant but was far away from the location where the explosion occurred. “Then we went outside and saw the smoke.”

Few details were available on the extent of the injuries or what

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