A 46-year-old double murder in Massachusetts has had its first break in decades.

Police have made an arrest in the West Springfield murders of 18-year-old Theresa Marcoux and 20-year-old Mark Harnish, who were found shot to death at a rest stop on the morning of Nov. 19, 1978. They were last seen alive at a friend's party two days before they were found.

Among the little evidence police found at the scene: a single fingerprint.

That fingerprint turned out to be one of the keys that led police to arrest Timothy Joley at his home in Clearwater, Florida, on Oct. 30, Hampden District Attorney Anthony Gulluni announced on Wednesday.

Joley, 71, was being held without bond on two murder counts in Florida's Pinellas County Jail and will be brought to Massachusetts in the coming weeks to face the charges.

No attorney was listed for Joley in court records. USA TODAY left a message at his home phone number.

Here's what you need to know about the cold case that devastated the lives of two families.

The importance of a single fingerprint

West Springfield Police Department and Massachusetts State Police never recovered a gun from the rest stop where Marcoux and Harnish were found. They concluded that the couple had been shot inside their 1967 green Dodge pickup truck, which remained at the scene, and that their bodies had been taken to a concrete retaining wall.

The only key piece of information investigators had was one latent fingerprint on the passenger vent window of the pickup truck, Gulluni said. The fingerprint did not belong to Marcoux or Harnish and for years, investigators were unable to find a match.

That was until recently, when the Hampden District Attorney’s Office got a tip from someone who provided Joley's name. Investigators found that Joley lived in West Springfield, Massachusetts, in November 1978, was a licensed gun owner and had bought a gun one month before the murders.

Two investigators with "extensive fingerprint experience" each concluded that the fingerprint found on the truck originated from Joley's left thumb, Gulluni said.

The Springfield District Court issued a two-count murder complaint and arrest warrant on Oct. 29 and Joley was arrested on Oct. 30.

As of Wednesday, Gulluni said investigators are unaware of a motive or relationship between Joley and victims. When asked whether Joley had any previous criminal charges, Gulluni said, "nothing of any real significance."

Who were Theresa Marcoux and Mark Harnish?

Marcoux and Harnish both attended but did not graduate from East Longmeadow High School in East Longmeadow, Massachusetts, according to 1978 reporting from the North Adams Transcript.

"Theresa ... was described as someone who loved to laugh and always had a smile on her face. Before her death, Theresa was working at a local hardware store as a clerk in their pet department," Gulluni said on Wednesday. "Mark ... was known as a quiet, polite young man who had been working at a car repair shop in town."

Investigators believe the pair had been living out of the 1967 green Dodge pickup truck, which was determined to be Harnish's.

Gulluni addressed the extended surviving family of Marcoux and Harnish at the news conference. (Their parents are no longer alive.)

"I admire and respect you for your patience, resolve and the faith that I know you've maintained over these many years," Gulluni said. "While we may have crossed a hill today and we can see justice, there are many more uphill battles ahead."

Greta Cross is a national trending reporter at USA TODAY. Follow her on X and Instagram @gretalcross. Story idea? Email her at [email protected].