GEDmatch’s terms of service in 2017 and early 2018 did not explicitly address law enforcement use of the database. The investigators who uploaded the Golden State Killer’s DNA profile — developed from the Ventura County rape kit among other crime-scene evidence — to GEDmatch were operating in a legal and ethical environment that had not yet produced clear guidelines for this kind of use. The method was novel. The database’s operators had not anticipated this specific use case when they designed the platform.
The upload produced matches to approximately ten to twenty people who shared segments of DNA with the crime-scene profile. None of those matches were close relatives. They were distant — cousins of various degrees, the kind of genetic relationships that are typically not known about by the people involved because the common ancestor is three, four, or five generations back.
This is where the genealogical work began. Rae-Venter and the investigative team used the distant relative matches as anchor points from which to build family trees outward and upward. If someone in the database shared a detectable DNA segment with the crime-scene profile, it meant they shared a common ancestor with the unidentified person. The task was to identify that common ancestor and then trace the descendants forward in time to find who among them, in terms of age, sex, and California residence in the relevant years, could be the person the investigators were looking for.
The genealogical work required going back, in some branches, to the level of great-great-great-grandparents. The research drew on public records available to genealogists: census records, birth and death certificates, marriage records, military records, newspaper archives, local history collections. It required building large family trees for each of the genetic matches, identifying all the individuals in those trees who could have been alive in California between 1974 and 1986, and systematically eliminating those who could be ruled out based on sex, age, or documented presence elsewhere.