A DNA sample found in the ditch where a 21-year-old woman was shot and left in 1975 has reinvigorated a nearly 50-year-old murder investigation.

Investigators "discovered an unknown DNA sample" from the scene where the body of 21-year-old JoAnn Bontjes was found in 1975, Martin County Sheriff Jeff Markquart said in a statement released Thursday. It's been submitted it to the state Bureau of Criminal Apprehension (BCA) for analysis.

"We are in the process of trying to determine whose DNA this is," the sheriff's statement continued.

Investigators have compared the sample with samples collected from "multiple people ... and everyone that has submitted a sample has been ruled out as being a contributor to our unknown DNA profile," Markquart said.

Bontjes was last seen on Oct. 2, 1975, about 12:30 a.m., when she left the American Legion club in Trimont. She was thought to be making the drive home to Sherburn, about 8 miles south. Sherburn is 20 miles west of Fairmont along Interstate 90.

The next day, a farmer spotted Bontjes' partly clothed body in a ditch near a cemetery roughly 4 miles south of Trimont. She had been shot in the head with a shotgun.

Two miles south of Trimont, law enforcement found her car along Hwy. 4. Her shoes were on the pavement, her purse and jacket were in the car, and the keys were in the ignition.

Tire tracks at the scene indicated another vehicle had pulled in front of her car, the Sheriff's Office said at the time.

Bontjes operated a beauty salon in Sherburn. The search began when she failed to show up for work the next day.

A few days after Bontjes was found dead, the Sheriff's Office disclosed reports from several beauticians in nearby Fairmont about what they described as sexually suggestive telephone calls from a male in the days leading up to the the body's discovery.

Anyone with information about this case is urged to contact Sheriff's Sgt. Matt Owens at 507-238-3167 or the BCA tip line at 877-996-6222.

The Associated Press contributed to this report.