For decades, it seemed that Lindy Sue Biechler’s killer would never be found.

The 19-year-old newlywed’s stabbing death in her Lancaster County apartment in 1975 had stunned her community and shattered her family. As years passed, police kept working the case, slowly eliminating suspects as technology improved.

Detectives on the scene had managed to save a sample of the killer’s DNA. But for decades it had no match in DNA databases of convicted criminals. That’s when CeCe Moore, a genetic genealogist at Parabon Nanolabs, a company that works with police to solve cold cases, decided to dig a little differently.

She and her colleagues in Reston, Va., identified the common ancestors of people who were partial matches to the DNA sample. They were all from a small town in Italy. And through generations and generations of genealogical records, authorities identified a prime suspect.

This week, Heather Adams, the Lancaster County district attorney, announced the arrest of David Sinopli, 68, a Lancaster native whose DNA, authorities say, matched the sample taken from the scene of Biechler’s murder nearly five decades before.

“It was a really good feeling to be able to get to this point,” Adams said. “It’s also just the beginning of the court process.”

Biechler’s was the oldest cold case in Lancaster County. The flower-shop clerk with long, flowing hair and a gentle smile had been married for just a year to Phillip Biechler; the couple lived together at an apartment complex in Manor Township. On the evening of Dec. 5, 1975, Lindy had deposited her and her husband’s paychecks at the bank and headed out for groceries.

She would never get to unpack them. Shortly after arriving at her apartment, Biechler was attacked in her living room. Her aunt and uncle, who’d been planning to visit that night to exchange recipes, found her just before 9 p.m. She had been stabbed 19 times with two knives — one taken from her own kitchen — and sexually assaulted.

Adams credited detectives at the scene in 1975 for securing DNA evidence — semen found on Biechler’s underwear. As technology improved, the killer’s DNA profile was submitted into CODIS, a national DNA database that includes samples of convicted criminals. It turned up no matches.

In 2019, the Lancaster County District Attorney’s Office formed a Cold Case unit, which took over the case. They enlisted Parabon.

Moore’s background is in theater and advertising; her interest in genealogy was…