Jim Bontjes and his brother-in-law crisscrossed the flatland of southern Minnesota for hours one early autumn day in 1975, looking for some sign of his little sister. JoAnn Bontjes had vanished a couple of days earlier, after an evening socializing with friends at the American Legion in Trimont.

Bontjes drove to abandoned farms, gravel pits, all over Martin County checking for any trace of JoAnn, until he came across a sheriff's squad car and an ambulance parked along a roadside. He knew his search was over. A half-century later, her killing is still unsolved, but a new DNA sample recovered from the crime scene could reinvigorate an investigation that never went far.

"The deputy sheriff told me it was her body. And that was it," Jim Bontjes said Friday, recalling a moment nearly 50 years ago. JoAnn Bontjes, who operated a beauty salon in nearby Sherburn, had been shot in the head with a shotgun.

It was Oct. 3, 1975, that a farmer found the body in a roadside ditch south of Trimont. Bontjes' parents both died not knowing what happened to her. Now, investigators "discovered an unknown DNA sample" from the scene where the body was found, Martin County Sheriff Jeff Markquart said in a statement released Thursday. It has 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 those 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 in the prepared statement. No one was available with the Sheriff's Office on Friday to elaborate on the makeup of the DNA or how it was found.

Jim and JoAnn Bontjes were two of four children raised in Sherburn by their parents, B.J. and Wilma. Jim Bontjes, now 74, said his brother died in 2007 and his sister in 2008. Their parents, Wilma and B.J. Bontjes, also did not live to know who took their youngest child.

As time passed and no leads emerged, "you just didn't bring it up," Jim Bontjes told the Star Tribune. Even now, his hope is muted. He's the only one left from the family of six.

"I have no way to know, one way or the other" whether a resolution will come, he said. "It's just that Mom and Dad would have loved to have answers. But they're gone, so I just keep my fingers crossed."

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

The next day, the farmer saw Bontjes' partly clothed body in a ditch near a cemetery roughly 4 miles south of Trimont.

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

The search began when Bontjes failed to show up at the beauty salon the next day.

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

Lisa Lenning said she and Bontjes were friends. The two were hanging out together at the Legion in Trimont early in the evening the day Bontjes was last seen alive.

"We had been together that afternoon and at the Legion," Lenning told the Star Tribune. She recalled nothing out of the ordinary before she headed home to Trimont.

"Then she didn't show up for work and didn't get home," Lenning said of her friend, who she'd known for about a year and a half. "Her sister started calling around and asking questions. ... I didn't have anything to tell her."

Lenning said an investigator came by her home "and asked a bunch of questions, and that was that."

For a while after Bontjes' death, "there were a lot of stressed-out people, including myself" and other women who traveled on Hwy. 4 between Sherburn and Trimont, Lenning said.

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.

Lenning now lives about 7 miles north of Trimont. She said she knows solving this murder mystery "would make a lot of people [satisfied]. Just closure, I'm trying to say."

The Associated Press contributed to this report.