The service can find the whereabouts of almost any cellphone in the country within seconds. It does this by going through a system typically used by marketers and other companies to get location data from major cellphone carriers, including AT&T, Sprint, T-Mobile and Verizon, documents show.

Between 2014 and 2017, the sheriff, Cory Hutcheson, used the service at least 11 times, prosecutors said. His alleged targets included a judge and members of the State Highway Patrol. Mr. Hutcheson, who was dismissed last year in an unrelated matter, has pleaded not guilty in the surveillance cases.

As location tracking has become more accurate, and as more people carry their phones at every waking moment, the ability of law enforcement officers and companies like Securus to get that data has become an ever greater privacy concern.