An explosion at a U.S. Steel plant in Clairton, Pa., on Monday injured dozens of people, the authorities said, drawing multiple emergency crews to what the authorities called a mass casualty incident.

“We’re up to about 15 ambulances,” with outside agencies assisting local fire and police departments, said Kasey Reigner, a public information officer with Allegheny County Emergency Services.

There were scant details available on the extent of the injuries and what caused the explosion at the plant near the Monongahela River south of Pittsburgh.

U.S. Steel did not immediately respond to a request for comment.

This is a developing story. Check back for updates.