But on Jan. 27, Ritterson died of prostate cancer at 77.
Assistant District Attorney Matthew Weintraub said last week the investigation remained active. A sample of Ritterson's DNA obtained legally after his death is being tested to see whether it matches hairs found with the victim's body. Such a test was unavailable to investigators in 1977, he said.
If the DNA matches, it will add to what Weintraub says is a collection of circumstantial evidence against Harry Ritterson: witness accounts that he visited the site where the body was found while his niece was still missing; a report that he wanted a romantic relationship with her; and conflicting statements he made last year to reporters about his whereabouts on the weekend of Shaun Ritterson's death.
"It's important to follow through and see if he did commit this murder," Weintraub said. "And if he didn't, to see who did."
The mystery of Shaun Ritterson's disappearance started on June 10, 1977, a Friday night.
Contemporary news reports and the search warrant filed in January seeking Ritterson's DNA provide a timeline of the night, which began with Shaun Ritterson bar-hopping with two friends. The three eventually ended up at Club Capri in Bristol Township.
Around 1 a.m., Shaun Ritterson left with a man described as in his early 20s. He was not believed to be Harry Ritterson; the victim's cousin Jack Lister, who worked at the bar and saw her leave, did not recognize the man.
Shaun Ritterson wasn't seen again until her body was found on Sunday in the woods on Buckingham Mountain.
Investigators began to focus on Harry Ritterson when they learned about his activities that weekend.
According to the investigation, on Saturday, Harry Ritterson proposed a trip to the Poconos with his girlfriend, Jackie LaSalle, and another friend, Raymond Albright.
But first, he wanted to stop at a spot where he regularly took friends and girlfriends to drink beer: the side of Buckingham Mountain.
He drove his 1973 Chevrolet Monte Carlo up the hill and parked only a few hundred yards from where his niece's body was later found. There, the three drank beer.
While there, he went to relieve himself at an outhouse that was even closer to the corpse.
Investigators believe he did that to see whether the body was still there.
"My true opinion is that he couldn't resist," Weintraub said.
The body was found by a father and son who were driving by.
Harry Ritterson consistently denied any involvement in the killing.
But his actions after the body was discovered drew scrutiny. Albright, for instance, told police Ritterson had asked him to say they had been together Friday night, which Albright said they hadn't been.
And Ritterson initially refused to take a polygraph test, saying he was worried about what the results might show.
In a 2012 interview with the Bucks County Courier Times, he seemed to contradict his earlier statements, saying he had visited Buckingham Mountain on his motorcycle "the night before" with a girlfriend. He did not specify whether he meant Friday - the night before he visited the mountain in his car - or Saturday, the night before the body was found.
He also said he knew his niece had a "secret lover" who was probably responsible for her death, another new admission.
There were no weapons found with the body, and none of Shaun Ritterson's organs had been disposed of there, leading police to suspect she was killed somewhere else.
The only substantial physical evidence was hair found at autopsy that was described in the search warrant as "microscopically dissimilar" from the victim's.
Weintraub said Harry Ritterson had agreed to a DNA test but never provided a sample. Authorities obtained it after his death.
Another circumstantial clue cropped up after investigators reopened the case.
According to the DNA search warrant, one of Shaun Ritterson's friends with her the night she disappeared told police she often complained about her uncle Harry, saying he had pursued her sexually.
Harry Ritterson told the Bucks newspaper he and his niece were close, calling her his "favorite." But there was no mention of any attempted sexual relationship.
Efforts last week to reach Ritterson's live-in girlfriend and his sister-in-law Nancy Ritterson, the victim's mother, were unsuccessful.
Weintraub acknowledges there are still many questions about Shaun Ritterson's death, including the huge void of time after she left the bar.
But the circumstantial evidence, he said, points to Harry Ritterson, and the investigation now hinges on the results of the DNA test, which he expects in the coming months.
If Harry Ritterson's DNA matches the hairs found with his niece's body, Weintraub said, that would be enough to close the case.
But if it doesn't, the search will continue, he said.
The DNA results "are going to give us very definitive facts to work with, where we'll either be able to implicate someone, or eliminate anyone else," he said. "If it ends up not being who we have reason to believe it is, then we'll reopen those avenues of investigation."
Contact Chris Palmer at 609-217-8305, firstname.lastname@example.org, or follow on Twitter @cs_palmer.