A suspect has been identified in the 1990 cold case murder of a Seattle teenager through the investigative tool of genetic genealogy, Washington state officials announced.

But the suspect, Robert Brooks, won’t stand trial, because he died in 2016 from natural causes, the Snohomish County Sheriff’s Office said Thursday.

Brooks has now been linked to the sexual assault and murder of 17-year-old Michelle Koski, who was found dead on Aug. 25, 1990 in Snohomish, the sheriff’s office said.

Brooks, then 22, had been released from prison four months before the killing, the sheriff’s office said. He was staying with family just a few blocks from where Koski lived, the sheriff’s office said.

Koski’s case had already been unsolved for 15 years when it was picked up by the cold case team in 2005, the sheriff’s office said. There wasn’t a DNA match from the crime scene to anyone in the CODIS law enforcement database, officials said, and over the next 10 years, several suspects were ruled by testing their DNA.

The case finally identified Brooks as a suspect by using genetic genealogy, which takes an unknown suspect’s DNA left at a crime scene and identifies it using his or her family members who voluntarily submit their DNA samples to a DNA database. This allows police to create a much larger family tree compared with using only databases like CODIS.

A forensic genealogist spent about one year building family trees using the unknown suspect’s DNA, which she eventually narrowed down to two brothers, officials said. Brooks’ DNA was then matched to DNA at the crime scene, officials said.