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

With cellphones ubiquitous, the police call phone tracing a valuable weapon in emergencies like child abductions and suicide calls and investigations in drug cases and murders. One police training manual describes cellphones as “the virtual biographer of our daily activities,” providing a hunting ground for learning contacts and travels.

But civil liberties advocates say the wider use of cell tracking raises legal and constitutional questions, particularly when the police act without judicial orders. While many departments require 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.

The internal documents, which were provided to The New York Times, open a window into a cloak-and-dagger practice that police officials are wary about discussing publicly. While cell tracking by local police departments has received some limited public attention in the last few years, the A.C.L.U. documents show that the practice is in much wider use — with far looser safeguards — than officials have previously acknowledged.

The issue has taken on new legal urgency in light of a Supreme Court ruling in January finding that a Global Positioning System tracking device placed on a drug suspect’s car violated his Fourth Amendment rights against unreasonable searches. While the ruling did not directly involve cellphones — many of which also include GPS locators — it raised questions about the standards for cellphone tracking, lawyers say.

The police records show many departments struggling to understand and abide by the legal complexities of cellphone tracking, even as they work to exploit the technology.

In cities in Nevada, North Carolina and other states, police departments have gotten wireless carriers to track cellphone signals back to cell towers as part of nonemergency investigations to identify all the callers using a particular tower, records show.

In California, state prosecutors advised local police departments on ways to get carriers to “clone” a phone and download text messages while it is turned off.

In Ogden, Utah, when the Sheriff’s Department wants information on a cellphone, it leaves it up to the carrier to determine what the sheriff must provide. “Some companies ask that when we have time to do so, we obtain court approval for the tracking request,” the Sheriff’s Department said in a written response to the A.C.L.U.

And in Arizona, even small police departments found cell surveillance so valuable that they acquired their own tracking equipment to avoid the time and expense of having the phone companies carry out the operations for them. The police in the town of Gilbert, for one, spent $244,000 on such equipment.

Cell carriers, staffed with special law enforcement liaison teams, charge police departments from a few hundred dollars for locating a phone to more than $2,200 for a full-scale wiretap of a suspect, records show.

A number of judges have come to conflicting decisions in balancing cellphone users’ constitutional privacy rights with law enforcement’s need for information.

In a 2010 ruling, the United States Court of Appeals for the Third Circuit, in Philadelphia, said a judge could require the authorities to obtain a warrant based on probable cause before demanding cellphone records or location information from a provider. (A similar case from Texas is pending in the Fifth Circuit.)

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