An explosion ripped through U.S. Steel’s Clairton Coke Works south of Pittsburgh on Monday morning, killing at least one worker and injuring dozens more, Allegheny County officials said. Two people were still unaccounted for as of late afternoon, and a Level 3 Mass Casualty Incident—signifying 20 to 100 potential victims—was declared. Emergency crews from local, state and federal agencies were continuing search-and-rescue operations for employees trapped beneath collapsed structures. The blast, reported around 10:51 a.m. EDT, sent a plume of black smoke over the Monongahela Valley, prompting the county health department to advise residents within a one-mile radius to stay indoors and keep windows closed while air-quality monitors assess soot and sulfur-dioxide levels. The Clairton facility, capable of producing more than four million tons of coke annually and employing roughly 1,300 people, is the largest coking plant in North America. Investigators are focusing on the plant’s 13/15 battery reversing room, where local media say the blast originated. Governor Josh Shapiro and U.S. Senator John Fetterman pledged state assistance and urged the public to avoid the area while the cause of the explosion is probed.