In DeAngelo’s case, investigators identified him as a priority candidate. They conducted surveillance. They waited for an opportunity to collect biological material from something he had used and discarded.
The specifics of the collection are documented in public reporting from the arrest period. Investigators collected material DeAngelo had left at a public location — identified in various accounts as a pizza restaurant he frequented or a Department of Motor Vehicles office. The material was submitted to a laboratory for DNA analysis. The DNA profile developed from the discarded material was compared against the Golden State Killer crime-scene profile.
It matched.
The match between DeAngelo’s discarded DNA and the crime-scene profile provided the direct biological evidence that the genealogical research had not and could not have provided on its own. The genealogical work had identified DeAngelo as a candidate. The discarded DNA comparison established that the candidate was the person whose DNA was at the crime scenes.
This sequence — genealogical lead followed by independent corroboration through direct DNA comparison — is the correct account of how the identification worked. The genealogy database did not identify DeAngelo directly. It identified distant relatives, whose family trees, when mapped, pointed toward him. The family-tree research confirmed he was in the right age, sex, location, and background category. The discarded DNA comparison then established the biological connection.
Paul Holes retired from the Contra Costa County District Attorney’s Office in late March 2018. He had spent decades on the case. On his last day before retirement, he drove by DeAngelo’s Citrus Heights home and watched the house. He chose not to approach or make contact, knowing that any premature action could alert DeAngelo and compromise the investigation. He retired without making the arrest he had worked toward for so long.