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. Some departments log dozens of traces a month for both emergencies and routine investigations.

With cellphones now ubiquitous, the police describe phone tracing as an increasingly valuable weapon in a range of cases, including emergencies like child abductions and suicide calls and investigations into drug cases, sex crimes and murders. One California police training manual describes cellphones as “the virtual biographer of our daily activities,” providing a rich hunting ground for learning someone’s contacts and travels.

But civil liberties advocates say the widening use of cell tracking raises legal and constitutional questions, particularly when the police act without judicial orders. While many departments require court warrants to use phone tracking in nonemergencies, others claim broad discretion to get the records on their own, according to 5,500 pages of internal records obtained by the American Civil Liberties Union from 205 police departments nationwide.
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