Under federal law, the carriers said they generally required a search warrant, a court order or a formal subpoena to release information about a subscriber. But in cases that law enforcement officials deem an emergency, a less formal request is often enough. Moreover, rapid technological changes in cellphones have blurred the lines on what is legally required to get data — particularly the use of GPS systems to identify the location of phones.

As cell surveillance becomes a seemingly routine part of police work, Mr. Markey said in an interview that he worried that “digital dragnets” threatened to compromise the privacy of many customers. “There’s a real danger we’ve already crossed the line,” he said.

With the rising prevalence of cellphones, officials at all levels of law enforcement say cell tracking represents a powerful tool to find suspects, follow leads, identify associates and cull information on a wide range of crimes.

“At every crime scene, there’s some type of mobile device,” said Peter Modafferi, chief of detectives for the Rockland County district attorney’s office in New York, who also works on investigative policies and operations with the International Association of Chiefs of Police. The need for the police to exploit that technology “has grown tremendously, and it’s absolutely vital,” he said in an interview.

The surging use of cell surveillance was also reflected in the bills the wireless carriers reported sending to law enforcement agencies to cover their costs in some of the tracking operations. AT&T, for one, said it collected $8.3 million last year compared with $2.8 million in 2007, and other carriers reported similar increases in billings.

Federal law allows the companies to be reimbursed for “reasonable” costs for providing a number of surveillance operations. Still, several companies maintained that they lost money on the operations, and Cricket, a small wireless carrier that received 42,500 law enforcement requests last year, or an average of 116 a day, complained that it “is frequently not paid on the invoices it submits.”

Image Credit... The New York Times

Because of incomplete record-keeping, the total number of law enforcement requests last year was almost certainly much higher than the 1.3 million the carriers reported to Mr. Markey. Also, the total number of people whose customer information was turned over could be several times higher than the number of requests because a single request often involves multiple callers. For instance, when a police agency asks for a cell tower “dump” for data on subscribers who were near a tower during a certain period of time, it may get back hundreds or even thousands of names.