A SeaTac man linked to the homicides of a British Columbia couple in 1987 through DNA and genealogical technology was found guilty Friday of  two counts of aggravated murder.

A Snohomish County jury deliberated for about two days before convicting William Earl Talbott II in the rape and murder of Tanya Van Cuylenborg, 18, and the murder of 20-year-old Jay Cook.

“It’s been such a long wait for all of us,” John Van Cuylenborg, Tanya’s older brother, said after the verdict in a video posted by The Daily Herald newspaper. “It feels great to have some answers. We don’t have all the answers, but we have a lot more than we had for 31 years.”

Cook and Van Cuylenborg set out for Seattle on an errand for her father on Nov. 18, 1987, taking the 4 p.m. ferry from Victoria to Port Angeles and buying a ferry ticket from Bremerton to Seattle.

The couple never appeared at the Seattle furnace-supply store to place an order for her father the next morning and didn’t arrive home that night. The next day, they were reported missing to police in the couple’s hometown of Saanich, B.C., a Victoria suburb.

Van Cuylenborg’s partially clothed body was found Nov. 24 in a ditch in Alger, Skagit County. Police said her hands were bound behind her with a plastic zip tie. She had been raped and shot in the head.

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The next day, police found the couple’s bronze 1977 Ford van in a parking lot in downtown Bellingham. Inside, they found Cook’s ID and plastic ties like the one used to bind Van Cuylenborg’s hands. Her wallet, keys to the van, a pair of surgical gloves and a partial box of .380-caliber ammunition were found under the back porch of a Bellingham tavern.

Two hunters found Cook’s body under a bridge south of Monroe on Nov. 26, 1987.

According to the criminal charges, Talbott lived in Woodinville from December 1984 to December 1993, seven miles from where Cook’s body was found.

Over the next three decades, detectives investigated hundreds of leads to no avail. But in 2017, Snohomish County sheriff’s detective Jim Scharf learned about Parabon Labs in Reston, Virginia, which was using a new method to extract more information from DNA samples.

The genealogy technique involves entering crime-scene DNA profiles into public genealogy databases, finding relatives of the person who left the DNA and building family trees that lead detectives to a suspect.

In April 2018, investigators from Snohomish and Skagit counties announced they had sent DNA from Van Cuylenborg’s killer to Parabon NanoLabs, which provided scientific approximations of what the killer might look like based on traits embedded in his genetic code.

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From there, a digital file containing DNA genotype data derived from the crime scene was uploaded to GEDmatch, a public genetic-genealogy website, and promising matches were found for two of Talbott’s relatives, according to the Snohomish County Sheriff’s Office.

Using the suspect DNA, a genealogist identified second cousins in the GEDmatch databank and from there, developed two family trees, one going back to the suspect’s paternal grandmother and the other to his maternal great-grandparents, according to the Sheriff’s Office.

The two family trees converged in a marriage, and Talbott — who was 24 at the time of the murders — is the only male carrier for the mix of DNA from the two families, the Sheriff’s Office said.

After the genealogist deduced Talbott’s identity through what she called “descendency research or reverse genealogy,” police collected DNA from a cup he had used.

Detectives had Talbott under surveillance on May 8, 2018, as he drove around in his work truck, charging papers say. At one point, he leaned out of his vehicle to check on something and apparently didn’t notice that a paper cup fell onto the road at West Marginal Way at South Spokane Street in Seattle, the charges say.

The Washington State Patrol crime lab confirmed that Talbott’s DNA found on the cup matched the DNA profile from the crime scene, according to the charges.

Genetic genealogy is the same technique that led to the arrest last year of Joseph James DeAngelo, the alleged serial rapist and killer known as the Golden State Killer in California.

After DeAngelo’s arrest, at least 50 other killings and rapes have been solved nationwide by using partial DNA matches to find suspects’ relatives, whose identities can lead to arrests.

But complaints about invasion of privacy have produced a backlash, leading the Florida-based GEDmatch to update its policy to establish that law enforcement only gets matches from the DNA profiles of users who have given permission. That closed off more than a million profiles. More than 50,000 users agreed to share their information — a figure that the company says is growing.

Talbott did not testify during the trial, and the jury rejected the suggestion from his lawyers that he had sex with Van Cuylenborg but did not kill her or her boyfriend. It’s still unknown how Talbott encountered the pair.

Information from Seattle Times archives and The Associated Press is included in this story.