An explosion Monday at a U.S. Steel coking plant near Pittsburgh has left people trapped under the rubble, with emergency workers on site trying to rescue them, an official said.

Emergency crews are on the scene of a reported explosion at the U.S. Steel Clairton Coke Works. There are multiple injuries, and multiple medical helicopters have been called to the scene. A mass casualty incident has been declared to allow for additional EMS resources from across the region to respond.