When detectives in a Phoenix suburb arrested a warehouse worker in a murder investigation last December, they credited a new technique with breaking open the case after other leads went cold. The police told the suspect, Jorge Molina, they had data tracking his phone to the site where a man was shot nine months earlier. They had made the discovery after obtaining a search warrant that required Google to provide information on all devices it recorded near the killing, potentially capturing the whereabouts of anyone in the area. Investigators also had other circumstantial evidence, including security video of someone firing...