HACKENSACK, N.J. — When FBI agents wanted to reconstruct the movements of a rogue New York City cop who staged a $1 million perfume heist in Carlstadt, N.J., last February, they turned to cell phone records to trace his steps.

Using a computer mapping program and “”call detail”” logs obtained from Sprint Nextel, agents plotted the locations of 42 cell sites in Bergen and Hudson counties and New York to track Kelvin L. Jones’ movements as the armed robbery plot unfolded. Jones was convicted last month.

Cellular tracking of criminals — including those like Jones who use prepaid mobile phones that can’t easily be traced because there is no subscriber contract — has become a cottage industry for the FBI.

The demand for cell site records has mushroomed as the ability to zero in on phones has become more and more precise, drawing criticism from civil libertarians and prompting some courts to take a new look at the legal ground rules for granting access to such data.

Prosecutors relied on similar historical cell data last year to link two Philadelphia men to a 2009 Pennsylvania home-invasion robbery that left a businessman dead after he was bound and beaten.

Cell site data also helped win the 2005 conviction of a Cliffside Park, N.J., man in the execution-style shooting of his best friend in Fort Lee four years earlier, placing the gunman in a Brooklyn neighborhood where the victim’s Lincoln Navigator was torched.

The ability to retrace Jones’ steps on the day of the Carlstadt robbery using cell records proved to be highly incriminating in conjunction with other evidence, especially for a target who, because of his experience and training, knew how to cover his tracks, said Assistant U.S. Attorney Christopher Gramiccioni, who prosecuted the case.

The FBI formed a dedicated unit, the Cellular Analysis and Survey Team, in late 2009 “”because the need has been overwhelming across the country for this type of evidence,”” FBI Special Agent William B. Shute testified at Jones’ trial in Newark last month. The unit provides technical assistance, case support and training to federal, state and local law enforcement officers around the nation, he said.

In densely populated North Jersey, Shute said, records routinely kept by cellular service providers can be used to pinpoint a targeted phone to within three-quarters of a mile of the cell towers that originated and terminated its calls.

Though a boon to law enforcement, the government’s method of obtaining historical cell site data — without any showing of probable cause that tracking the location of a cell phone will turn up evidence of a crime — has come under fire from privacy advocates, civil libertarians and some federal judges in recent years.

The Stored Communications Act of 1986 allows prosecutors to obtain court orders for cell site logs merely by showing that the tracking information is “”relevant and material”” to an ongoing criminal probe. That is a much lower burden than the probable cause standard required under the Fourth Amendment, which guarantees the right of the people to be secure against unreasonable searches and seizures.

Kevin Bankston, a senior staff attorney with the Electronic Frontier Foundation, said the government’s reliance on warrantless cell tracking is cause for alarm.

“”People should be concerned because, whether they realize it or not, they’re carrying a tracking device in their pocket,”” Bankston said. “”And phone companies are collecting data about where your phone is located, even when you’re not using it, that can reveal a really intimate portrait of how you spend your days and nights, where you go, who you associate with.

“”This is a new cache of highly sensitive information,”” he said. “”And we think that being able to go into the past and see everywhere you’ve been based on your cell phone’s location is just as invasive as, say, wiretapping your phone calls, which clearly is protected against by the Fourth Amendment.””

Based on current cell phone and texting patterns, cell site data for a typical adult user will reveal between 20 and 55 location points a day — enough to plot his or her movements hour by hour, a federal magistrate judge in Houston noted in October in denying a bid for cell records.

In 2008, a U.S. magistrate in Philadelphia, acting in a drug-trafficking probe, refused to order a provider to disclose a customer’s historical cell tower data without a judicial finding of probable cause. In a unique show of support, all of the other magistrates in the district joined in the judge’s opinion.

The government appealed all the way up to the Third U.S. Circuit Court of Appeals in Philadelphia. Rather than definitively settling the Fourth Amendment issue, the three-judge panel sent the case back to the lower court for additional fact-finding and analysis “”that balances the government’s need (not merely desire) for the information with the privacy interests of cell phone users.””

The appeals court concluded that prosecutors can continue to obtain cell location data under the less-stringent relevancy standard laid out in the Stored Communications Act. But the court also held that judges can require a warrant showing probable cause, although they should use that option sparingly.

The government argued warrantless tracking is permitted because Americans enjoy no “”reasonable expectation of privacy”” when it comes to the whereabouts of their cell phones.

The court, however, agreed with EFF’s lawyers that cell customers do not voluntarily convey location information, especially when they receive calls, because it is unlikely they are aware their providers collect and store historical location data.

Much about the government’s cell-tracking activities remains cloaked in secrecy, as are details about the CAST unit.

An FBI spokeswoman in Washington declined to provide any information about CAST’s operations, noting the agency does not discuss investigative techniques.

In his testimony in Newark, Shute said the practice of tracking targets based on their cell use has become widespread.

“”I use it every day and have used it to find hundreds of people,”” he said. “”The agents that I have trained have used it to find thousands of people.””

In New Jersey, the first glimpse into the practice came in 2009 when the American Civil Liberties Union released documents showing that federal prosecutors were granted mobile phone location data 79 times after the Sept. 11, 2001, terrorist attacks without first obtaining a warrant based on probable cause.

In other applications since November 2007, investigators were permitted to track users of newer GPS-equipped cell phones in real time without warrants, even when the phones were not in use, the ACLU said.

The ACLU raised the specter of “”Big Brother.”” But Republican Gov. Chris Christie, who served as U.S. attorney in New Jersey during most of that period before he stepped down to run for governor, defended the practice, noting the applications were all court-approved.

The call detail records on which the analysis is based are similar to the monthly bills customers get. They list such information as date, time, duration, the calling party and party called and the digits dialed

But these records contain additional information identifying the region, tower and a cell sector number that further narrows the location of the phone — data that Shute said “”is like a fingerprint on the cell phone network.””

The ACLU and the EFF say they don’t object to law enforcement using cell-tracking records as long as they get a warrant first.

“”The fundamental problem here is the federal law governing this area was written in 1986 when cell phones were as large as your head,”” says Catherine Crump, a staff attorney with the ACLU in New York. “”Here in 2011 everyone is struggling — prosecutors, defense attorneys, courts — to figure out how to apply this extremely outdated statutory scheme to technology that simply wasn’t contemplated at the time.””

Ultimately, adds the EFF’s Bankston, the uncertainty will continue until either the Supreme Court rules on the issue or Congress steps in and clarifies the law.