A man on trial for rape and double murder is thought to be the first suspect to be found using the new forensic technique of genetic genealogy.
Truck driver William Earl Talbott II is one of dozens of men police have arrested for unsolved crimes in the past year using the technology.
Genetic genealogy sees investigators enter crime-scene DNA into a public database linked to popular sites used by people to research their family trees.
The publicly available information can reveal a relative, then suspects are identified by meticulously researching birth and death certificates, local papers and social media.
A former police officer accused of being the Golden State Killer, one of the most notorious serial killers in US history, was charged with 13 counts of murder in 2018 after being linked to the crimes using the same technique.
It has been criticised by privacy campaigners who say police use of the databases should be restricted.
Canadians Tanya Van Cuylenborg, 18, and her boyfriend, 20-year-old Jay Cook disappeared in November 1987 during an overnight trip from their hometown of Saanich, British Columbia, to Seattle.
Talbott’s DNA was allegedly found on Miss Van Cuylenborg’s trousers and body.
His defence argues that the presence of his DNA does not necessarily make him the killer.
“The police used this as nothing more than any other tip, which they followed up with traditional investigative techniques,” defence lawyer Rachel Forde said.
“DNA on the hem of one of the victim’s pants doesn’t tell you who killed her and why.”
A week after their last sighting Ms Van Cuylenborg’s body, which had been shot in the back of the head, was found in an embankment in a rural part of Seattle.
Two days after that, hunters found her boyfriend 60 miles (95km) away. Mr Cook had been strangled with twine and dog leashes.
The case remained cold until in 2017, when Snohomish County sheriff’s detective Jim Scharf learned about the new DNA processing method to extract more information from samples using the public genealogy database GEDmatch.
A sample was taken from Ms Van Cuylenborg’s trousers, which had been found in the couple’s van after their deaths, and a family tree was established.
From that, it was determined the source must be a male child of William and Patricia Talbott.
Their only son is William Talbott II, who is now 56 but was 24 at the time of the killings and lived close to where Mr Cook’s body was discovered.
Talbott was followed by investigators, who tested DNA left on a coffee cup used by him – which was found to be a match.
His palm also matched a print on the rear door of the van.
John Van Cuylenborg, the victim’s brother, said: “For the computing power and DNA technology to advance together to make this kind of thing possible, it was fantastic,” he said.
However, friends of Talbott wrote to court, attesting that he is a kind, gentle and helpful person.
As the technique has grown, GEDmatch has changed its policy to require people to opt-in to allow law enforcement to be able to access to their DNA profile, which closed off more than a million profiles to officers.