Updated

At least 10 people were injured, including three critically, in an explosion Thursday at a U.S. steel castings plant, officials said.

The blast happened in the morning in the cleaning-and-finishing department at the American Steel Foundries plant in Illinois. 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 hospitals, Gateway spokeswoman Kate Allaria said.

Two of them were listed in critical condition later Thursday at Mercy Hospital in Creve Coeur, spokeswoman Bethany Pope said. It wasn't immediately clear where the other worker was being treated.

Federal workplace safety officials are investigating the cause of the blast. Calls and emails to the business went unanswered Thursday.

Robert Lott, a worker at the plant and president of the United Steelworkers Local 1063, told the Belleville News-Democrat that at least two of those injured may have suffered broken legs and internal injuries. He said he believed the blast was caused by a gas leak.

"I could see flames, initially, and then the visibility went down to zero from the dust that fell from the rafters," said Lott.

Right said the union was sending an investigator to the Granite City plant "to make sure we learn the lessons and really understand what happened."

"At this point everything is pretty sketchy, and we really don't like to comment on causes until we have them tied down," he said.