A Washington state man has been found guilty in the brutal 1987 double murder of a young Victoria couple in Western Washington.

A jury began deliberating Tuesday afternoon in the trial of William Talbott II and delivered the verdict Friday morning. He wasn't arrested until last year, when authorities said they used genetic genealogy to identify him as the person who left his DNA on one of the victims.

Prosecutors say on Nov. 18, 1987, 18-year-old Tanya Van Cuylenborg and her boyfriend, 20-year-old Jay Cook, drove a van from Saanich to Seattle for an overnight trip.

When they didn't return, their families began a frantic search for them, including renting a plane to try to spot the copper-colored Ford van they had been driving.

About a week later, Van Cuylenborg's body was found down an embankment in rural Skagit County, north of Seattle. 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, which is about 60 miles (95 kilometres) from where his girlfriend was discovered. He had been beaten with rocks and strangled with twine and two red dog collars, authorities said.

The couple's van was found in Bellingham, Washington, near a bus station. Van Cuylenborg's pants were in it; investigators found semen on the hem, and said it matched that on her body.

Snohomish County deputy prosecutor Matt Baldock said in court that their families may never know specifics of what happened. But he argued that the question of who did it was answered.

Sheriff's detective Jim Scharf fought back tears on the witness stand as he recalled a phone call from Parabon NanoLabs, which uses a new DNA processing method to extract more information from samples.

With the help of a genealogist, the lab had found a man whose genetic profile matched DNA from a crime scene.

“What was that name?” the deputy prosecutor asked.

“William Earl Talbott II,” Scharf told the jury in Snohomish County Superior Court.

The defence suggested Talbott's semen could have ended up at the crime scenes - on pants in the van, and on Van Cuylenborg - from consensual sex.

Defence attorney Rachel Forde said police developed “tunnel vision” over the years, clearing people as suspects because their DNA didn't match the semen.

“They never stopped to consider that perhaps the person who left the DNA was not the murderer,” Forde said.

Jon Scott described his 56-year-old client as a “blue-collar guy” who had worked in construction and as a truck driver and lived a “quiet, unremarkable life.”

Talbott of SeaTac, Washington, chose not to testify.

The gun used to kill Van Cuylenborg was never recovered. Scharf testified that Talbott's parents lived 7 miles (11 kilometres) from the rural bridge where Cook's body was dumped.