Billboards are being used to encourage tips from the public after Vienna Township authorities say a rock thrown from a highway overpass smashed a car's windshield and killed a 32-year-old man Wednesday night.
Police say Kenneth White of Mount Morris was traveling in the car southbound on Interstate 75 when he was hit by the rock.
Investigators say four other vehicles were struck with large rocks or concrete chunks.
Crime Stoppers of Flint and Genesee County is encouraging anyone with information to come forward. A $2,500 reward is being offered.