(WPXI) – Emergency crews are on the scene of an explosion at the U.S. Steel Clairton Coke Works. At least one person is dead, multiple people were injured in the explosion and authorities say they believe at least two people remain unaccounted for and crews are searching for people who may be trapped in the rubble. A mass casualty incident has been declared to allow for additional EMS resources from across the region to respond. Multiple medical helicopters were called to the scene. The Allegheny County Health Department said out of an abundance of caution, they are advising residents within one mile of the plant to remain indoors, close all windows and doors, set HVAC systems to recirculate and avoid activities that draw in outside air, such as using exhaust fans. The Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) confirmed on social media that they are assisting at the plant.