Mary Schlais, of Minnesota, was found dead in Dunn County, Wisconsin, on Feb. 15, 1974.
Schlais, an accomplished artist, was believed to be hitchhiking from her hometown of Minneapolis to the art show in Chicago when she was killed, authorities said. A witness reported seeing a suspect and suspect vehicle at the time.
Over the decades, there were many tips, leads and interviews conducted by multiple law enforcement agencies. There were also several items of evidence examined and re-examined over the years as there were technological advances in DNA -- but no viable suspects were ever identified.
A key piece of evidence -- a stocking cap -- was left at the crime scene, but the hairs on the hat had no matches in the Combined DNA Index System, the law enforcement database known as CODIS, according to the sheriff's office.
Then, last year, the Dunn County Sheriff's Office partnered with Ramapo College of New Jersey to try to solve the case using investigative genetic genealogy.
The school's program is headed by Professor David Gurney who works with students and others to help solve cold cases.
After interviewing and collecting DNA from multiple potential relatives, authorities were led to 84-year-old Jon Miller, the criminal complaint states.
Miller's DNA was found to be a match on Nov. 4 -- which would've been Schlais' 76th birthday, police noted.
Authorities charged Miller with first-degree murder on Thursday, according to court records. He is in custody, awaiting extradition to Wisconsin.
On Thursday, investigators met with Miller, who initially denied knowing about Schlais' killing. When told of the DNA evidence, Miller admitted to picking up Schlais while she was hitchhiking and asking for "sexual contact," the complaint states. He allegedly told investigators that when she said no, he grabbed a knife he had stowed in the car and fatally stabbed her in the back.
Schlais had multiple stab wounds when she was found dead, according to the complaint, including defensive wounds on her hands.
According to the complaint, Miller said he pulled off the highway and tried to hide Schlais' body in a snowbank, but got scared when a car drove by and left the area.
Miller also admitted the stocking cap found at the scene was his, the complaint said. He was arrested following the interview with investigators and is being held at the Steele County Jail.
"He did inform us that as soon as he had opened the door, he knew why we were there," authorities said.
(ABC News and CNN Wire contributed to this report.)
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