Department of Public Safety spokesman Warren Strain said investigators were called about 5:30 a.m. Saturday to U.S. Highway 61 in the Boyle community just south of Cleveland.
The body of Shelby Harmon was found at the site. Strain told The Associated Press that investigators have questioned a man but nobody had been charged by Wednesday. He said the investigation is in its early stages.
Strain would not say how many vehicles struck Harmon. However, Sgt. Ben Williams, spokesman for the Highway Patrol region that includes Bolivar County, later said in a separate interview that she was struck by three.
Bolivar County Coroner Nate Brown said no autopsy was performed because a county sheriff's deputy who's also investigating the case did not request one. The sheriff's department referred all calls to the Highway Patrol.
Strain said evidence has been sent to state crime laboratories in Batesville and Jackson.
Harmon graduated from Cleveland High School in 2010 and earned an associate's degree from Mississippi Delta Community College.