ST. LOUIS – At least 10 people were injured, including three critically, in an explosion Thursday at a steel castings plant in southwestern Illinois, officials said.

The blast happened shortly after 8 a.m. in the cleaning-and-finishing department at the American Steel Foundries plant in Granite City, just northeast of St. Louis. Roughly 800 people work at the plant, which produces railcar undercarriages and related components, said Mike Right, the United Steelworkers union’s health, safety and environment chief.

Seven workers treated for smoke inhalation at Gateway Regional Medical Center were released later Thursday, while three others were in critical condition and flown by helicopter to other Missouri hospitals, Gateway spokeswoman Kate Allaria said.