On the afternoon of Nov. 18, 1987, Jay Cook and Tanya Van Cuylenborg left Saanich, B.C., their hometown, to pick up some furnace equipment in Seattle for Cook’s father. Saanich and Seattle are a little more than 100 miles apart, but the trip takes almost five hours: a ferry into the U.S. across the Strait of Juan de Fuca, another across Puget Sound, and between them a winding coastal drive through evergreen forests and fishing towns. The young couple planned to make an overnight jaunt of it. A blurry photo snapped at the time shows them beside the bronze Ford van they took. Van Cuylenborg, 18, holds a walk-like-an-Egyptian pose; Cook, two years older and a head taller, looks off to the side half-smiling, his dark hair falling over one eye.

The next day they didn’t show up at the heating-supply store, nor did they return home that night as planned. On Nov. 24, Van Cuylenborg’s partially clothed body, hands bound by a zip tie, was found in a roadside ditch 75 miles north of Seattle. She had been raped and shot in the back of the head. Two days later, hunters spotted Cook’s body, wrapped in a torn blue blanket, under a bridge in a small town outside Seattle. He’d been beaten over the head with a rock and strangled; a pack of cigarettes was stuffed in his mouth.

Your DNA Is Out There. Do You Want Law Enforcement Using It?

Detectives from Snohomish and Skagit counties’ sheriff’s departments had a few things to go on. There was the blue blanket, which didn’t belong to either of the victims, and there was Van Cuylenborg’s missing Minolta camera—the lens turned up in a pawnshop in Portland, Ore. The killer left behind extra zip ties and a box of .380-caliber bullets, and, in what seemed like a taunt, the surgical gloves he’d used to ensure that there were no fingerprints. Investigators carefully gathered up and tagged what they found. They also gathered something else, a new type of evidence.

Only weeks before, a Florida serial rapist had become the first American convicted on the basis of DNA left at a crime scene. In the years since, DNA has come to be seen by investigators and crime-drama fans alike as unique in its infallibility and its power to turn infinitesimal human traces into telltale clues. But then as now, DNA testing had its limitations: Samples could be degraded or contaminated, the results misinterpreted by technicians. And if the DNA left at the scene of the crime didn’t exactly match another sample—one from an existing criminal database or one taken from a suspect—then it really wasn’t much of a clue. In the Cook-Van Cuylenborg case, there was DNA (authorities haven’t specified the exact nature), but there were no matches. The new technology led nowhere.