In recent years, law enforcement officials have turned to cellular technology as a tool for easily and secretly monitoring the movements of suspects as they occur. But this kind of surveillance - which investigators have been able to conduct with easily obtained court orders - has now come under tougher legal scrutiny.

In the last four months, three federal judges have denied prosecutors the right to get cellphone tracking information from wireless companies without first showing "probable cause" to believe that a crime has been or is being committed. That is the same standard applied to requests for search warrants.

The rulings, issued by magistrate judges in New York, Texas and Maryland, underscore the growing debate over privacy rights and government surveillance in the digital age.

With mobile phones becoming as prevalent as conventional phones (there are 195 million cellular subscribers in this country), wireless companies are starting to exploit the phones' tracking abilities. For example, companies are marketing services that turn phones into even more precise global positioning devices for driving or allowing parents to track the whereabouts of their children through the handsets.

Not surprisingly, law enforcement agencies want to exploit this technology, too - which means more courts are bound to wrestle with what legal standard applies when government agents ask to conduct such surveillance.

Cellular operators like Verizon Wireless and Cingular Wireless know, within about 300 yards, the location of their subscribers whenever a phone is turned on. Even if the phone is not in use it is communicating with cellphone tower sites, and the wireless provider keeps track of the phone's position as it travels. The operators have said that they turn over location information when presented with a court order to do so.

The recent rulings by the magistrates, who are appointed by a majority of the federal district judges in a given court, do not bind other courts. But they could significantly curtail access to cell location data if other jurisdictions adopt the same reasoning. (The government's requests in the three cases, with their details, were sealed because they involve investigations still under way.)

"It can have a major negative impact," said Clifford S. Fishman, a former prosecutor in the Manhattan district attorney's office and a professor at the Catholic University of America's law school in Washington. "If I'm on an investigation and I need to know where somebody is located who might be committing a crime, or, worse, might have a hostage, real-time knowledge of where this person is could be a matter of life or death."

Prosecutors argue that having such information is crucial to finding suspects, corroborating their whereabouts with witness accounts, or helping build a case for a wiretap on the phone - especially now that technology gives criminals greater tools for evading law enforcement.

The government has routinely used records of cellphone calls and caller locations to show where a suspect was at a particular time, with access to those records obtainable under a lower legal standard. (Wireless operators keep cellphone location records for varying lengths of time, from several months to years.)

But it is unclear how often prosecutors have asked courts for the right to obtain cell-tracking data as a suspect is moving. And the government is not required to report publicly when it makes such requests.

Legal experts say that such live tracking has tended to happen in drug-trafficking cases. In a 2003 Ohio case, for example, federal drug agents used cell tracking data to arrest and convict two men on drug charges.

Mr. Fishman said he believed that the number of requests had become more prevalent in the last two years - and the requests have often been granted with a stroke of a magistrate's pen.

Prosecutors, while acknowledging that they have to get a court order before obtaining real-time cell-site data, argue that the relevant standard is found in a 1994 amendment to the 1986 Stored Communications Act, a law that governs some aspects of cellphone surveillance.

The standard calls for the government to show "specific and articulable facts" that demonstrate that the records sought are "relevant and material to an ongoing investigation" - a standard lower than the probable-cause hurdle.

The magistrate judges, however, ruled that surveillance by cellphone - because it acts like an electronic tracking device that can follow people into homes and other personal spaces - must meet the same high legal standard required to obtain a search warrant to enter private places.

"Permitting surreptitious conversion of a cellphone into a tracking device without probable cause raises serious Fourth Amendment concerns, especially when the phone is monitored in the home or other places where privacy is reasonably expected," wrote Stephen W. Smith, a magistrate in Federal District Court in the Southern District of Texas, in his ruling.

"The distinction between cell site data and information gathered by a tracking device has practically vanished," wrote Judge Smith. He added that when a phone is monitored, the process is usually "unknown to the phone users, who may not even be on the phone."

Prosecutors in the recent cases also unsuccessfully argued that the expanded police powers under the USA Patriot Act could be read as allowing cellphone tracking under a standard lower than probable cause.

As Judge Smith noted in his 31-page opinion, the debate goes beyond a question of legal standard. In fact, the nature of digital communications makes it difficult to distinguish between content that is clearly private and information that is public. When information is communicated on paper, for instance, it is relatively clear that information written on an envelope deserves a different kind of protection than the contents of the letter inside.

But in a digital era, the stream of data that carries a telephone conversation or an e-mail message contains a great deal of information - like when and where the communications originated.

In the digital era, what's on the envelope and what's inside of it, "have absolutely blurred," said Marc Rotenberg, executive director of the Electronic Privacy Information Center, a privacy advocacy group.

And that makes it harder for courts to determine whether a certain digital surveillance method invokes Fourth Amendment protections against unreasonable searches.

In the cellular-tracking cases, some legal experts say that the Store Communications Act refers only to records of where a person has been, i.e. historical location data, but does not address live tracking.

Kevin Bankston, a lawyer for the Electronic Frontier Foundation, a privacy advocacy group that has filed briefs in the case in the Eastern District of New York, said the law did not speak to that use. James Orenstein, the magistrate in the New York case, reached the same conclusion, as did Judge Smith in Houston and James Bredar, a magistrate judge in the Federal District Court in Maryland.

Orin S. Kerr, a professor at the George Washington School of Law and a former trial attorney in the Justice Department specializing in computer law, said the major problem for prosecutors was Congress did not appear to have directly addressed the question of what standard prosecutors must meet to obtain cell-site information as it occurs.

"There's no easy answer," Mr. Kerr said. "The law is pretty uncertain here."

Absent a Congressional directive, he said, it is reasonable for magistrates to require prosecutors to meet the probable-cause standard.

Mr. Fishman of Catholic University said that such a requirement could hamper law enforcement's ability to act quickly because of the paperwork required to show probable cause. But Mr. Fishman said he also believed that the current law was unclear on the issue.

Judge Smith "has written a very, very persuasive opinion," Mr. Fishman said. "The government's argument has been based on some tenuous premises." He added that he sympathized with prosecutors' fears.

"Something that they've been able to use quite successfully and usefully is being taken away from them or made harder to get," Mr. Fishman said. "I'd be very, very frustrated."