Over four decades after a young couple was discovered dead in their Montana home, authorities revealed Monday they finally tracked down their killer with the help of a genealogy database.

The Yellowstone County Sheriff’s Office announced that Linda and Clifford Bernhardt, both 24, were killed at their Billings-area home in 1973 by a former co-worker of Linda’s.

“Today we can tell you that based on the evidence collected on the scene, which includes biological evidence and all the reasonable inferences taken from this evidence, we have determined that Cecil Stan Caldwell, a former coworker of Linda Bernhardt at Ryan’s Inc. is the person responsible for the deaths of Linda and Clifford Bernhardt,” Yellowstone County Sheriff Mike Linder told reporters. Caldwell died in 2003.

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Clifford Bernhardt was a concrete worker and Vietnam veteran and his wife worked at a grocery distribution warehouse. They had been married several years and moved into a new house just weeks before they were killed.

Photos of Linda and Clifford Bernhardt, who were killed in 1973, are displayed at a press conference at the Yellowstone County administrative offices in Billings, Montana on Monday, March 25, 2019. Yellowstone County Sheriff Mike Linder, pictured at right, says authorities have identified the couple’s now-deceased killer. (AP Photo/Matthew Brown)

Linda Bernhardt had been bound and sexually assaulted before her death, and authorities used psychologists to try to build a profile of the suspect, according to the Billings Gazette.

Linder said that evidence collected at the scene of the home, including biological evidence, tied Caldwell to the killings, although he did not identify a motive. The sheriff, however, believed that Linda was targeted by Caldwell.

Caldwell had no criminal record and died in 2003 at the age of 59, according to his obituary in the Billings Gazette.

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Authorities had conducted hundreds of interviews over the years, even bringing in a psychic at one point as part of their search for clues.

Investigators remained stymied until 2004, when DNA was discovered on evidence gathered at the crime scene, according to the sheriff. But comparing that DNA against an FBI database of known criminals yielded no results, leaving authorities frustrated yet again.

In 2012, the county formed a cold case unit, which made the murders a priority. Three years later, the unit enlisted a Reston, Virginia technology company, Parabon NanoLabs, to analyze the DNA by comparing it to genetic samples available through a public genealogy database.

Scott Goodwin, a volunteer with the cold case unit who helped with the investigation, told the Associated Press that he and others involved were unwilling to let it go.

“We were obsessed with it,” Goodwin said. “These are two young people who didn’t deserve what happened to them. They didn’t do anything. They came home on a Tuesday night and they were murdered.”

After running it through a public genealogy database, officials ultimately narrowed the list of suspects to Caldwell and his brother, who is still alive and living outside the area, according to Vince Wallis, a former detective captain with the sheriff’s office who now works for the Billings Police Department.

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Wallis said that after DNA was obtained voluntarily from the brother, it was analyzed by the Montana State Crime Lab to eliminate him as a suspect. That left only Caldwell, Wallis said.

“It’s the kind of police work that we are blessed to have in Montana every day,” Montana Attorney General Tim Fox told reporters.

The families of the victims issued a statement at the news conference thanking the sheriff’s office for its work, but made no further comment and asked for privacy.

The Associated Press contributed to this report.