A Montana cold-case investigative unit has teamed up with a Virginia DNA lab to crack the mystery of a double slaying 45 years ago, authorities say.

Linda and Clifford Bernhardt, both 24, were found dead in their Billings home on Nov. 7, 1973, by Linda Bernhardt's mother after the younger woman failed to show up for work. Yellowstone County Sheriff Mike Linder on Monday identified the killer as Cecil Stan Caldwell, a co-worker of Linda Bernhardt who died in 2003 at the age of 59.

Caldwell had no arrest record, and authorities likely will never know the motive for the slayings, Linder said.

"Anything we have would be an assumption," he said. "Linda was probably the target, but we will never know."