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CHICAGO — A woman was critically injured in a highway shooting involving an off-duty police officer.

The incident happened after 10 p.m. Thursday in the northbound lanes of the Stevenson Expressway near Archer Avenue. The woman was transported to Northwestern Memorial Hospital in critical condition.

Illinois State Police say it may have been a case of road rage. The chain of events leading up to the shooting are unclear at this time.

Police said two cars were involved: the woman shot and another person were in one vehicle; the off-duty officer was in a second vehicle.

The Civilian Office of Police Accountability is urging any witnesses to contact the agency at (312) 746-3609.

The investigation is ongoing.