Relatives of the woman believed to have been killed by Mr. Day could not be found, Sergeant Homrock said.

“There are so many unanswered questions,” he said. “We are never going to know now.”

In the Billings case, Ms. Reich described a long process of coming to grips with her sister’s violent end, which she learned about as a 16-year-old high school sophomore. After the murders, her family, which included two brothers, scattered to different cities and states where no one knew about them. “Our lives were shattered,” she said.

Ms. Reich, now 61, said she and her family had been hoping for justice as detectives worked on the case. There were no DNA matches within a national criminal justice database, Sheriff Mike Linder of Yellowstone County said in an interview. The county cold case unit’s work with the genealogy technology last year eventually led to Mr. Caldwell, who had no previous criminal record.

“He was not really on the radar,” Sheriff Linder said.

But with the suspect dead, he and the team were unable to answer remaining questions. Why were pairs of Ms. Bernhardt’s underwear missing? What was Mr. Caldwell’s motive? As a former grocery warehouse colleague of Ms. Bernhardt, had he been infatuated with her?

Why had Mr. Caldwell signed the condolence books at his victims’ funerals?

Sheriff Linder said that when Mr. Caldwell’s family was informed that he was the suspect, there was “maybe some disbelief, maybe some not too surprised.”

The Bernhardts were childhood sweethearts and had been married a few years before they were killed. An obituary of Mr. Caldwell in The Billings Gazette, by contrast, describes a full life.

He had been married with two adopted children at the time of the Bernhardt killings in 1973, according to the obituary. He and his wife divorced five years after the murders. He remarried in 1979, and become a stepfather to the children of his second wife, Nancy. He had been working for the city and for a mail contractor when he died in 2003, it said.