Emergency crews were responding Monday morning to a reported explosion at U.S. Steel’s Clairton Coke Works, authorities said.

Allegheny County Emergency Services received a call for an “ongoing situation” at the plant for a potential mass casualty incident, county spokeswoman Kasey Reigner said. One person was unaccounted for and several were injured, authorities said.

Allegheny County dispatch said police, fire and emergency services crews were responding, and several people had been transported from the scene. Dispatch said the initial calls about the incident came in about 10:50 a.m.

The Allegheny County Health Department advised everyone living within 1 mile of the plant to remain inside, close all windows and doors and set their HVAC systems to recirculate, out of an abundance of caution. They also advised against using devices that draw in outside air, such as exhaust fans.

The county’s air quality monitors have not detected levels of air pollutants above federal standards, but are continuing to monitor the situation, the department said in a news release.