William Talbott is set to be sentenced Wednesday in the murders of Jay Cook and Tanya Van Cuylenborg.

Retired Snohomish County detective Rick Bart displays a group of zip ties during the trial for William Talbott II on June 19. Jurors didn’t know about the DNA evidence on the zip ties on June 28 when they found him guilty of murdering Jay Cook and Tanya Van Cuylenborg. (Andy Bronson/The Herald via AP, Pool file)

EVERETT — Detectives found the strips of sturdy white plastic at all three crime scenes in late 1987.

There were zip ties in a field alongside a bridge near the body of Jay Cook, 20, south of Monroe.

There were zip ties along the road near the body of Tanya Van Cuylenborg, 18, north of Mount Vernon.

There were zip ties in Bellingham, in the big bronze Ford van the couple drove around Puget Sound on what was to be an overnight trip to Seattle.

On one plastic tie in the van, a crime lab found DNA from three people.

Now, a new lab report, unsealed weeks after he’d already been found guilty of double murder, provides even more DNA evidence linking William Talbott II to the crimes. Odds of a mismatch were 1 in 90 million, according to the Washington State Patrol report.

The jury knew nothing about the additional genetic evidence on the zip ties when they found the 56-year-old SeaTac trucker guilty on June 28. Other evidence presented at the trial had convinced the jurors of Talbott’s guilt beyond a reasonable doubt.

Minutes later, jurors met in private with the lead detective and prosecutors, who revealed pieces of evidence that had not been presented at trial.

Learning about the DNA on the zip ties meant the difference between being able to sleep at night or continuing to toss and turn over what-ifs, two of the jurors told The Daily Herald.

“That would’ve been incredibly powerful to know, because when you have as little as we had, anything was big,” a third juror said of the additional evidence. “We learned that afterwards, and for some — especially those who were just emotionally having a hard time with the gravity of the decision — it was helpful to be able to have that conversation.”

The breakthrough arrest in the landmark case came more than 30 years after the killings, with the help of a new tool known as forensic genealogy. The Snohomish County Sheriff’s Office worked with a private lab and a genealogist to build a family tree for the suspect, based on crime scene DNA and a public ancestry website, GEDMatch.

It led them to Talbott, who had grown up in the Woodinville area, about seven miles from the bridge where Cook’s battered body was found.

Talbott’s semen had been left on a pair of Van Cuylenborg’s pants in the van, and on her body.

She had been shot in the back of the head.

Cook had been beaten with rocks and strangled.

In the past year, arrests have been made in dozens of other cold cases nationwide through forensic genealogy, but Talbott’s was the first to go to trial.

Jury selection began June 11, in Snohomish County Superior Court.

It wasn’t until more than a week later, 12:30 p.m. on June 19, that a state crime lab supervisor, Lisa Collins, sent an urgent email to deputy prosecutor Matt Baldock.

“Hey Matt, I have something very important that I need to discuss with you,” she wrote. “Can you call me at your earliest convenience?”

Collins had come across old notes about a prior, inconclusive DNA test on the zip tie. It was too complex and degraded for meaningful comparison. But since that test, the lab had started to use new software that could decipher individual profiles of DNA from a mixed sample of several people.

Baldock told her to retest it as soon as possible, “because of the potential for exculpatory evidence.” In other words, if it showed someone else’s genetic profile, it would not be fair to withhold that information from the defense. The lab finished the test by the next morning.

The result identified Talbott as one of the three people whose DNA was on the zip tie.

A second sample on the tie appeared to be from Van Cuylenborg, with the odds of a false positive being 1 in 24,000.

Tests suggested the third person’s DNA came from an unknown female. (At trial, Collins testified mysterious traces of female DNA had ended up on another piece of evidence, a blue blanket that covered Cook’s body. It turned out the genetic profile matched a lab employee, who had worked in the same area where the blanket was examined.)

Out of caution, prosecutors did not present the zip tie DNA evidence to the jury last month.

Talbott’s lawyers acknowledged during the trial that he had sexual contact with Van Cuylenborg, and argued it was consensual, but did not offer any specifics about what happened. His attorneys argued that sexual contact did not prove murder.

Jurors said they could not find a reasonable, innocent explanation for the evidence against Talbott.

Defense attorney Jon Scott asked the judge for a new trial in a motion earlier this month. He argued a prosecutor’s closing argument shifted the burden of proof to the defense, and focused on an appeal to jurors’ emotions, rather than the actual evidence.

“Of the physical evidence admitted,” Scott wrote, “nearly none even related to Mr. Talbott.”

The defense also argued comments made by jurors, in a Daily Herald article published after the verdict, suggested the jury ignored the court’s instructions.

Juror No. 2, for example, drew a map of Western Washington from memory to retrace the couple’s route in 1987. This was after the judge declined the jury’s request for a map of the region, because an overview hadn’t been submitted as an exhibit at trial. The state argued that common knowledge of the jurors’ home state is not outside evidence.

Also, the prosecutor noted the defense was arguing based on “statements attributed to three jurors in a newspaper article — double hearsay.” And even if the judge was inclined to consider statements in a news article, the state argued, case law doesn’t support overturning a verdict based on a jury’s mental processes, motives or beliefs. In some of his final words to the jury, the prosecutor emphasized the weight of the case, but told the jurors it should not influence their decision, “except insofar as it makes you careful.”

It’s not uncommon for defense attorneys to seek a new trial after a guilty verdict.

In interviews with The Daily Herald, jurors said they came to believe Cook and Van Cuylenborg were killed soon after their ferry docked in Seattle. They planned to sleep in the van, then deliver a $750 money order to an appliance business in the morning.

Instead, they vanished before sunrise. Passersby found their bodies days later, more than 60 miles apart.

Despite eight days of testimony, jurors were left with many questions about exactly what happened.

To them, it made no sense that Van Cuylenborg would consent to sex with a random stranger, in a foreign country, while on her menstrual period, then come across another stranger, who killed her and her boyfriend, while leaving behind no DNA.

Superior Court Judge Linda Krese is set to consider the defense motion for a new trial Wednesday.

If it’s denied, Talbott will be sentenced afterward.

He faces one possible punishment for two counts of aggravated first-degree murder: life in prison.

Caleb Hutton: 425-339-3454; chutton@heraldnet.com. Twitter: @snocaleb.