On a clear day, from the middle of a well-trafficked stretch of reinforced concrete that spans the Snoqualmie River in Washington state, you can make out the hulking, ice-covered flanks of Mount Rainier. Locals bring their dogs to the so-called High Bridge to swim on their lunch breaks; high school kids in oversized hoodies pick their way through the raspberry thickets looking for a shady spot to light up. There are no memorials, no plastic flowers, no signs to mark the occasion when, nearly 32 years ago, somebody dumped the body of a young man at the base of this bridge, half-wrapped in a powder-blue blanket, plastic cords still cutting into his neck, and a pack of Camel Lights shoved down his throat.

Jay Cook’s battered, barely 21-year-old body was discovered on Thanksgiving Day in 1987, two days after his 17-year-old girlfriend Tanya Van Cuylenborg was found dead in a ditch one county over. She had been shot in the head and was believed to have been sexually assaulted. The young Canadian couple had been reported missing for nearly a week, after they failed to return to their home on Vancouver Island, British Columbia, from an overnight errand to buy furnace parts in Seattle. Despite investigating hundreds of leads and subjecting crime scene samples to new DNA technologies as they arrived in the ’90s and 2000s, police never arrested any suspects. For more than three decades the case went unsolved.

Then in May of last year, Washington officials announced a breakthrough. Snohomish County Sheriff’s Detective Jim Scharf stood at a podium and told the reporters assembled that they had, at long last, arrested a suspect: a balding, middle-aged man who had grown up in the area named William Earl Talbott II. “He was never on any list law enforcement had, there was never a tip providing his name,” said Scharf. “If it hadn’t been for genetic genealogy, we wouldn’t be standing here today.”

At the time, the statement probably didn’t mean much to most people beyond the Cook and Van Cuylenborg families. Talbott, who would later plead not guilty, was only the second person to be fingered using a new forensic technique known as genetic genealogy. It involves creating DNA profiles like the kind you’d get through 23andMe or Ancestry from crime scene samples and looking through public genealogy websites for matches, which could surface family members that lead to new suspects. The first, a California man accused of being the notorious Golden State Killer, had been taken into police custody mere weeks before.

Since then, the technique has been used to help identify suspects in at least 50 cases, even as critics warn it could mean the end of genetic privacy. Police departments and the Federal Bureau of Investigation have formed their own dedicated family tree-building units; multiple companies have launched lucrative genetic genealogy services, and it’s all happening with no federal or state laws to rein it in. Now the Talbott case, which begins this week, will be the first to put genetic genealogy on trial.

At stake is more than justice for Cook and Van Cuylenborg. The trial’s outcome could result in legal precedents that could determine the future of one of the most powerful and invasive tools for finding people to ever fall into the hands of law enforcement.

On Tuesday morning, in Snohomish County’s largest courtroom, jury selection begins for what will likely be a monthlong double-homicide trial. It will be up to the 12 chosen to weigh the evidence against Talbott—including the science behind genetic genealogy, which is sure to be hotly contested. Last week the defense filed pretrial motions arguing that the genealogy evidence should be inadmissible, on which Judge Linda C. Krese is expected to make a ruling later this afternoon.