EVERETT — A SeaTac man was found guilty Friday of the brutal 1987 double murder of a young Canadian couple in Western Washington.

Snohomish County Superior Court jurors deliberated for 2 1/2 days before reaching their verdict against William Earl Talbott II in the trial that hinged on 32-year-old DNA evidence and cutting-edge genealogical technology.

Talbott was charged with the killings after authorities said they used genetic genealogy to identify him as the person who left his DNA on the clothing of one of the victims. The practice identifies suspects by entering crime-scene DNA profiles into public databases that people have used for years to fill out their family trees.

The prosecution had argued that Talbott’s DNA, plus evidence like blood and Talbott’s palm print found in the couple’s van, proved beyond a reasonable doubt that he raped 18-year-old Tanya Van Cuylenborg and killed her and her boyfriend, 20-year-old Jay Cook.

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Prosecutors said Van Cuylenborg and Cook were killed after they left their hometown near Victoria, British Columbia, for what was supposed to be an overnight trip to Seattle in November 1987.

About a week later, Van Cuylenborg’s body was found down an embankment in rural Skagit County. She was naked from the waist down and had been shot in the back of the head. Hunters found Cook dead two days later in brush near a bridge over the Snoqualmie River in Monroe. He had been beaten with rocks and strangled with twine and two red dog collars, authorities said.

The case went cold for decades after detectives investigated hundreds of leads and tested the DNA against criminal databases, to no avail. But it was reopened after CeCe Moore, a genealogist, used the public genealogy database GEDmatch to find distant cousins of the person who left the DNA. She built a family tree and determined the source must be a male child of William and Patricia Talbott, of Monroe.

William Talbott II, now 56, was their only son. He was 24 at the time of the killings and lived in Woodinville, not far from where Cook’s body was found.

The defense called only one witness – an investigator who testified about various addresses where the defendant has lived. The idea was to establish the fact Talbott was not trying to hide from anyone in the decades that have passed since the 1987 murders.

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On cross examination, the prosecutor pressed the defense investigator about Talbott’s address at the time of the murders, and the investigator conceded that previous testimony indicated Talbott had been living in Woodinville at that time.

The defense didn’t dispute Talbott’s semen was found on Tanya’s pants, but they argued that it didn’t prove rape or murder. They called the state’s evidence into question, claiming his DNA – the result of a consensual act – is the only link between Talbott and the couple.