Holes used DNA recovered from a crime scene to find the killer’s great-great-great grandparents, who lived in the early 1800s. Branch by painstaking branch, he and a team created about 25 family trees containing thousands of relatives down to the present day.

One fork led to a 72-year-old retiree who was quietly living out his golden years in the Sacramento suburb of Citrus Heights. Holes was intrigued after learning the man was a disgraced cop who had bought guns during two bursts of activity by the killer…

The role of genetics in the case is well known, but this account reveals for the first time the massive scope, intricate science and sheer doggedness of the effort to catch one of the nation’s worst serial predators.

Initial DNA work identified distant relatives — not a suspect. Holes said a team of five investigators spent four months building out family trees, name by name. They pored over census records, newspaper obituaries, gravesite locaters, and police and commercial databases to find each relative and, ultimately, DeAngelo.