EVERETT — In a unanimous decision Thursday, the state Supreme Court reinstated two aggravated murder convictions for William Talbott II, in the landmark cold case that put “forensic genealogy” in front of a jury for the first time in the United States.

The case is now expected to return to the state Court of Appeals to address other legal questions raised by the defendant.

After 30 years of dead ends, public DNA ancestry databases and genealogical research led to a breakthrough in the case of slain Canadian couple Jay Cook, 20, and Tanya Van Cuylenborg, 18. A well documented investigation, arrest and trial culminated in a Snohomish County jury convicting Talbott, now 59, of killing the pair in 1987.

But legal arguments over one juror’s perceived bias led the state Court of Appeals to overturn the convictions in late 2021. Snohomish County prosecutors appealed that ruling to the state’s highest court. None of the arguments at issue centered on the pioneering DNA sleuthing.

Cook and Van Cuylenborg left Vancouver Island in a bronze Ford Club Wagon van to run an errand for the Cook family business on Nov. 18, 1987. They never arrived at their destination south of downtown Seattle. Passersby found their bodies days later, separately, along rural roads about 65 miles apart, on the week of Thanksgiving. The van had been abandoned farther north in downtown Bellingham. Inside was a ticket for the Bremerton-Seattle ferry stamped 10:16 p.m. Nov….