MT. VERNON, Ill. (AP) — A state trooper was wounded and a motorist killed early Tuesday in an exchange of gunfire along an interstate highway in southern Illinois, police said.

Illinois State Police said preliminary findings indicate that troopers were responding to help a motorist at about 3 a.m. along Interstate 64 when an altercation happened. Police said the motorist displayed a firearm and exchanged gunfire with troopers, leaving the suspect dead and a 16-year-veteran trooper hospitalized with non-life-threatening injuries.

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The shooting happened near Mt. Vernon, the Jefferson County seat, about 80 miles (129 kilometers) southeast of St. Louis, closing the interstate’s eastbound lanes for more than three hours.

State police said the agency’s own internal investigators are handling the case, and no more details were available to be released.