Law enforcement tracking of cellphones, once the province mainly of federal agents, has become a powerful and widely used surveillance tool for local police officials, with hundreds of departments, large and small, often using it aggressively with little or no court oversight, documents show. 

 

The practice has become big business for cellphone companies, too, with a handful of carriers marketing a catalog of “surveillance fees” to police departments to determine a suspect’s location, trace phone calls and texts or provide other services.