Explosion at Pennsylvania Steel Mill

An explosion at a U.S. Steel plant in Clairton, Pa., on Monday killed at least one person and injured dozens more, with some still trapped under rubble, the 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.