NEW YORK (NYTIMES) - On Dec 5, 1975, an aunt and uncle of Ms Lindy Sue Biechler went to her apartment in Manor Township, Pennsylvania, eager to exchange recipes with their 19-year-old niece, who had returned from getting groceries about two hours earlier.

On her front door, they saw blood.

Inside, they were startled by bloodstains near the entrance way and splotches of blood on the carpet, the authorities said.

Between the living room and dining room was Ms Biechler, lying on her back and bleeding from multiple parts of her body, a kitchen knife protruding from the left side of her neck.

Someone had stabbed Ms Biechler, a recently married flower shop clerk, 19 times, killing her, the authorities said that day.

The identity of the killer would elude them for 46 years.

But on Monday (July 18), the Lancaster County District-Attorney's Office and the Manor Township Police Department said they had arrested David Sinopoli, 68, of Lancaster County, Pennsylvania, a day earlier on homicide charges. The arrest closed a case that had haunted the county for more than four decades and frustrated local detectives who had investigated dozens of tips over the years to no avail.

"This has been a never-ending pursuit of justice for Lindy Sue Biechler," Ms Heather Adams, the district-attorney in Lancaster County, said at a news conference.

It was not immediately clear if Sinopoli had a lawyer. Phone calls placed on Tuesday to a number listed as belonging to him were not immediately returned.

In February, the authorities followed Sinopoli - who had lived in the same four-unit apartment building as Ms Biechler, and had recently been identified as a suspect - into Philadelphia International Airport.

They obtained his DNA from a coffee cup he had used and thrown into a rubbish bin before boarding a flight, Ms Adams said. From there, investigators matched his DNA with samples collected from Ms Biechler's underwear.

"This case was solved with the use of DNA and, specifically, DNA genealogy," Ms Adams said. "And, quite honestly, without that, I don't know that we would have ever solved it."

The authorities did not share a motive, but evidence at the scene suggested it was related not to a robbery but possibly to a sex crime.

Ms Adams said Ms Biechler was found with her jeans unbuttoned with the zipper down, exposing her underwear, which contained Sinopoli's semen.

The case joins a catalogue of others in recent years that have been solved through DNA testing, a powerful tool in courtrooms across the country that has both led to people being exonerated of wrongful convictions and helped investigators find the killers in cold cases from decades ago.

Ms Biechler's case was the oldest unsolved homicide investigation in Lancaster County, Ms Adams said, underscoring the years of futile efforts and faulty leads that left investigators confounded by the mystery surrounding her killing. Dozens of people had been cleared in the case through blood-type tests, DNA examinations or other evidence.

"Lindy Sue Biechler was on the minds of many throughout the years," Ms Adams said. "Certainly, law enforcement has never forgotten about her."

None of the tips that investigators received mentioned Sinopoli, whom Ms Adams said continued to live in the Lancaster County area since the killing. PennLive.com reported that Sinopoli had worked as a press operator for more than 40 years.

On the day that Ms Biechler was killed, she travelled from the flower shop where she worked to her husband's workplace to get his pay cheque, which she would deposit in a bank later that afternoon. Then she shopped for groceries.

Her aunt and uncle arrived at her apartment at 8.46pm, indicating that she was killed in a two-hour window when no one else was home, police said.

On Monday, the authorities said they had informed Ms Biechler's husband of Sinopoli's arrest.

"It's very meaningful to us, to be able to provide some sense of relief to the victims," Ms Adams said.