(CNN) A free-to-use, publicly accessible genealogy and DNA database helped bring police a step closer to the man they believe is the Golden State Killer, a key investigator in the case told CNN.

Paul Holes, a recently retired investigator with California's Contra Costa County District Attorney's Office, said he took crime-scene DNA -- believed to be that of the culprit -- and entered the profile into the online Florida-based GEDmatch database.

GEDMatch is a site where people enter their own DNA profiles or genealogical data -- such as those you can get from DNA tests through paid services such as Ancestry -- and try to find familial matches with other users.

Holes said that when he entered the crime-scene DNA profile, more than 100 users matched as a distant relative, possibly as close as a third cousin. To use GEDmatch, users agree to make their information public and attach at least an email address to their profile.

Holes said investigators contacted "one or two people" as they tried to find, from the distant relatives, a pool of people who could have been the Golden State Killer of the 1970s and '80s.

Read More