Deputies in this corner of western Oregon outside ultraliberal Portland used to track down criminals the old-fashioned way, faxing caught-on-camera images of a suspect around the office in hope that someone might recognize the face. Then, in late 2017, the Washington County Sheriff's Office became the first law enforcement agency in the country known to use Amazon's artificial-intelligence tool Rekognition, transforming this thicket of forests and suburbs into a public testing ground for a new wave of experimental police surveillance techniques. Almost overnight, deputies saw their investigative powers supercharged, allowing them to scan for matches of a suspect's face across...