For the past 30 years, evidence found at the scenes of the killings of Jay Cook and Tanya Van Cuylenborg was mostly confined to a blue blanket wrapped around Cook’s body, an abandoned bronze 1977 Ford Club Wagon and, crucially, the killer’s DNA.

The couple was from Saanich, British Columbia, and on the evening of Nov. 18, 1987, they were traveling in the Cook family van to Gensco Heating in Seattle to pick up a part for Cook’s father. They were last seen purchasing a ticket around 10 p.m. in Bremerton, Wash., to board a ferry to Seattle, but they never made it there. Several days later, 18-year-old Van Cuylenborg’s body was found partly clothed, dumped in a ditch in a wooded area in Skagit County, Wash.  She had been raped, police said. Cook’s body was found near the Snoqualmie River.

Since 1987, police had received more than 300 names from tipsters who thought they had information about the couple’s alleged killer. William Earl Talbott II was not on that list.

But then investigators ran the DNA from the scene through a genealogy website. They turned up two second cousins of Talbott, which led them to him.

And now they have charged Talbott, 55, with murder, saying his DNA profile found through his ancestors this month matches the DNA left at the crime scene 31 years ago.

“It’s the genetic genealogy that was the key tool that got this case resolved,” Detective Jim Scharf of the Snohomish County Sheriff’s Office, who sought the DNA technology and has spent 13 years studying this case, said at a Friday news conference. “Had law enforcement never had access to genetic genealogy, I don’t think this case ever could have been solved.”

Talbott pleaded not guilty to the first-degree murder charges on Friday. He is charged only in Van Cuylenborg’s death, but Scharf says he anticipates Talbott also being charged in Cook’s. An attorney for Talbott could not immediately be reached for comment.

Talbott’s arrest marks the latest case in which detectives have nabbed a suspected killer using controversial familial DNA methods, taking advantage of public genealogy websites — in this case, GEDMatch — to identify the suspect’s distant relatives and begin mapping his family tree. Last month, California detectives revealed that they used the same website to track down relatives of Joseph James DeAngelo, the suspected Golden State Killer who allegedly committed a dozen murders and 45 rapes across California in the 1970s.

GEDMatch said after DeAngelo’s arrest that it did not know law enforcement was using its website to track down suspects, but said its policy has always been to “inform users that the database could be used for other uses.”

The high-profile nature of the case led to a debate about the ethics of using these public DNA databases for police investigations. Some experts told The Washington Post that the most serious concern was that police could be infringing on the privacy of innocent people, who use the databases to research their family histories. Millions of unsuspecting Americans have plugged their spit, and thus their DNA, into these genealogy websites, the experts said, probably without the faintest idea that police might one day use it to track down possible criminals — their relatives.

University of British Columbia professor Wendy Roth explains genetic ancestry tests and how these tests influence how people understand race. (YouTube/Peter Wall Institute for Advanced Studies)

But on Friday, representatives from the DNA technology company Parabon NanoLabs said that privacy concerns of this nature were based on “misconceptions about the process and technology.”

Parabon assisted the Snohomish County and Skagit County sheriff’s offices in developing the DNA profile of the man police believe killed Van Cuylenborg and Cook. Using DNA found at the crime scene, Parabon genealogists created a “genotype file” for the suspected killer — containing his distinct genetic material — and uploaded it to GEDMatch. Steven Armentrout, chief executive of Parabon, said that the file was “set to private,” and that the entire process aligned with GEDMatch’s rules.

“At every turn, the contents of this file were not visible,” Armentrout said. “The site only makes it possible to compare DNA between two people and assess the degree of their genetic relationship. No GEDMatch users have had access to this file.”

His explanation did not address the central worry of critics that the DNA of GEDMatch customers is being used without their knowledge or permission.

CeCe Moore, a Parabon genealogist, said at the news conference that she used the alleged killer’s genotype file to identify relatives who “shared promising amounts of DNA” with him on the GEDMatch database. Those relatives were at the “second-cousin level,” she said. From there, she started filling in a family tree, going all the way back to great-grandparents.

“Then, I would start building it forward in what’s called reverse genealogy,” she said. “We’re looking for living people who could fit the profile of the suspect.”

Finally, she said, she reached a point where two family trees with DNA matching that of the suspected killer converged through a marriage. The couple only had one son.

“That led us to really only one person who could carry this mix of DNA,” Moore said.

Police now say that one person is William Earl Talbott II.

The Snohomish County Sheriff’s Office announced in April that it was partnering with Parabon to seek a DNA match for the killer through online genealogy. Once Parabon pulled Talbott’s name, investigators began following him, waiting for an opportunity to collect DNA from him to be cross-checked with the DNA left at the crime scene, just as investigators in the Golden State Killer case did.

Finally, Talbott dropped a paper cup on the side of the street. Investigators grabbed it and submitted it to the Washington State Patrol crime lab for testing. It was a match, they said.

Talbott was arrested outside his workplace in Seattle last week, Scharf said. He said Talbott apparently has been working as a truck driver, doing local jobs, for the past 20 years and has never been married. He would have been 24 at the time of the killings and living in Woodinville, Wash., in 1987. His parents lived seven miles from where Cook’s body was found, investigators say. Scharf said police “have no idea” what Talbott’s motive could have been or even how he and the victims were connected. But he said police are confident that he is the right suspect.

Family members of the victims attended Friday’s news conference to express “feelings of relief, joy, of great sorrow” knowing a suspect has been arrested, as Cook’s mother, Lee, put it.

“For 31 years, we have waited in hope for a day like this, a day like this for our family — the Cook family and the Van Cuylenborg family,” she said. “But how could we have known that the day would be so bittersweet. On the one hand, we are close to closure. On the other, we’re still at a loss. And I don’t have my only son, Jay.”