A genetic genealogy search for a match to a crime scene profile identifies eight relatives, one of whom is almost certain to be the perpetrator. A geofence warrant returns an anonymized list of four cell phones present at a series of bank robberies. A facial-recognition software analysis of a surveillance video generates fifteen persons of interest. By now, these scenarios are commonplace. Equally as commonplace are the two strands of case law and scholarship that have unfurled around them. What is missing, however, is an account of the middle. All too overlooked is a third tapestry, woven from both of these threads. Specifically, technological searches have dramatically increased both the frequency with which law enforcement confronts a closed universe of suspects rather than a single suspicious target, as well as the probability that the true perpetrator of a crime can be found by engaging in invasive technological searches within that closed universe. This Article is the first to identify and define closed universe searches as (1) searches of a small pool of persons connected by happenstance to a crime; (2) one of whom is almost certainly the perpetrator, but the rest of whom are equally certainly innocent; (3) using new technological tools that can identify the perpetrator from within the pool with certainty or near certainty. Applying this new concept, it then considers how Fourth Amendment doctrine has or might resolve closed-universe-search questions—questions like how intrusively police can investigate the suspects in that closed universe; how transparent investigative actions must be; and how accountable police are to rules designed to prevent abuse, misuse, or excess.

Forensic DNA databases have received an inordinate amount of academic and judicial attention. From their inception, numerous scholars, advocates, and judges have wrestled with the proper reach of DNA collection, retention, and search policies. Central to these debates are concerns about racial equity in forensic genetic practices. Yet when such questions arise, critics typically just…