This image from video provided by KMOV-TV in St. Louis shows an injured worker being helped into a helicopter Thursday outside the American Steel plant in Granite City, Ill. where an explosion injured at least 10 people, three critically.

ST. LOUIS - At least 10 people were injured, including two 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. The explosion occurred after gas caught fire in the finishing area of the plant, which makes railcar components, though what caused the ignition wasn't known, Granite City Fire Captain Bert Houston said late Thursday night.

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 flown by helicopter to other Missouri 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. Houston said he was aware of only two workers being critically injured.

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, who said he was about 100 yards from where the blast happened.

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.