For almost five decades, the killing of 19-year-old Lindy Sue Biechler — a newlywed found stabbed to death on the floor of her Lancaster County, Pa., apartment — had stumped authorities. They’d chased scores of tips, interviewed as many as 300 people, launched a task force, presented the case to crime experts and, as the years dragged on without answers, even tried consulting psychics.

What finally led to an arrest in the county’s oldest cold case was the discarded coffee cup — and genetic genealogy. Investigators zeroed in on Sinopoli after a researcher at Reston, Va.-based Parabon NanoLabs determined through DNA evidence that whoever killed Biechler probably had ancestors from a small Italian town called Gasperina. The researcher, CeCe Moore, flagged Sinopoli as a person of interest after poring over newspaper archives and historical records.

A decades-old killing may have been solved with new forensic tech and DNA from a conch shell

After the detectives’ stealth mission at the airport, DNA from the coffee cup was compared against DNA found on Biechler’s underwear. It was a match, Lancaster County District Attorney Heather Adams said. Authorities made their long-awaited arrest Sunday morning; Sinopoli, a onetime resident of Biechler’s apartment complex, was taken into custody and was being held without bail.

“This case was solved with the use of DNA and, specifically, DNA genealogy,” Adams said during a Monday news conference. “And quite honestly, without that, I don’t know that we would have ever solved it.”

She added: “The reality is that David Sinopoli was not on our radar.”

The gruesome killing happened the evening of Dec. 5, 1975, a Friday. Biechler’s aunt and uncle had stopped by her Manor Township apartment to exchange recipes. But when they arrived at the building, they found what Adams said “can only be described as a horrific scene.” Biechler lay on the living room floor, her jeans unbuttoned and her body covered with 19 stab wounds.

Bags of groceries sat on the dining room table. The young wife had been unloading…