A spokesman for an auto parts supplier says four people have been injured in an explosion at the company’s plant in Blacksburg.

Jim Burke, spokesman for Federal-Mogul in Southfield, said the injured were employees of a local contractor that was conducting routine duct work cleaning. He said there was no word on the cause of the explosion, which occurred shortly after 9:30 a.m. Friday.

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Eric Earnhart, a spokesman for Roanoke Carilion Hospital, said two critically injured patients were flown to Wake Forest University Baptist Medical Center in Winston-Salem, N.C.   George Ward, lead security officer at Lewis Gale Montgomery Hospital, said two patients were treated there for relatively minor injuries.

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The Blacksburg plant is one of 107 facilities operated by Federal-Mogul worldwide. It specializes in bearings.

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(The Associated Press contributed to this report.)