About 100 million Americans carry smartphones capable of emitting location data almost continuously. Even some less-sophisticated devices have such capacity, as do the navigation systems in automobiles and some laptop computers. (Seong Joon Cho/Bloomberg)

The rapid spread of cellphones with GPS technology has allowed police to track suspects with unprecedented precision — even as they commit crimes. But the legal fight is only now heating up, with prosecutors and privacy activists sparring over rules governing the use of powerful new investigative tools.

The U.S. Court of Appeals for the 6th Circuit stirred the debate last week when it supported police use of a drug runner’s cellphone signals to locate him — and more than 1,000 pounds of marijuana — at a Texas rest stop. The court decided that the suspect “did not have a reasonable expectation of privacy” over location data from his cellphone and that police were free to collect it over several days, even without a search warrant.

The decision riled civil libertarians, who warned that it opened the door to an extensive new form of government surveillance destined to be abused as sophisticated tracking technology becomes more widely available. On Monday, six days after the appeals court ruling, the U.S. attorney in Arizona cited it in defending the use of cellphone location data to help arrest a suspect accused of tax fraud.

“We’re looking at the very frightening prospect of an excessive degree of government intrusiveness in our personal lives,” said Gerald L. Gulley Jr., the Knoxville, Tenn., lawyer who represented the drug-running suspect at the Texas rest stop. “I don’t think that people who go out and buy cellphones necessarily contemplated . . . the degree of intrusion in their personal life.” Gulley says he’ll appeal the case.

Many legal experts expect the issue eventually to find its way to the Supreme Court, which touched on it in a January ruling that police violated the rights of an alleged D.C. drug dealer by placing a tracking device on the underside of his car.

About 100 million Americans carry smartphones capable of emitting location data almost continuously. Even some less-sophisticated devices have such capacity, as do the navigation systems in automobiles and some laptop computers. Worldwide, 154 million smartphones were shipped to consumers in just the past three months, according to International Data, a market analysis firm. (The Global Positioning System functions often can be switched off, but that deactivates some phone features.)

Changing technology has long strained the legal strictures of the Fourth Amendment, whose prohibition on “unreasonable” searches and seizures was born of 18th-century law and guides the legal standards for when police can tap phones, use tracking devices and monitor a suspect’s Internet activity.

Cellphones always have been trackable to some degree, as users moved among towers that carried the signals necessary to make the devices work, creating an electronic record in the process. But GPS technology is far more sophisticated, narrowing locations typically to within a few feet. Many smartphones relay location data to central servers throughout the day, as users check traffic, search for nearby restaurants or scan weather maps.

Combined with information from toll booths, credit card machines and security cameras, people in highly wired nations often move within a web of data that can allow governments to pinpoint individual movements down to the second.

The location data become even more potentially valuable when associations among people can be mapped, as they are on social media networks. A British research team tracking 25 volunteers in a Swiss town used GPS data, text messages and calling history to pinpoint current movements and predict where individuals were likely to be 24 hours in the future.

Researcher Mirco Musolesi, who teaches computer science at the University of Birmingham, said police could use location data to track suspects, predict crime hot spots, even anticipate political protests if enough potential participants are known. Downloading all the data on a single cellphone tower, as can be done easily with current technology, could allow police to identify those present at a demonstration.

Musolesi said the researchers in their experiment had to sign waivers promising to not use the data to determine where the 25 volunteers lived. “It’s just ethics that stops us doing this, as the data are there,” he said. “You can . . . construct information about behavior — if he’s in a movie theater, find out what he’s seeing. It’s quite fine-grained, so you can track shops. There are big privacy implications.”

The Supreme Court’s decision in January was narrow — dealing only with the issue of the police accessing the suspect’s car without a valid warrant — and left open the broader question of what rules guide government collection of the personal location data.

The appeals court went further in its ruling last week, noting that previous court cases have established that information legitimately in the public domain — such as an individual’s movements on public roadways — can be gathered by police in many situations. Improving technologies, the court said, are permitted to assist police investigations, even if criminals are not aware of the latest techniques.

“When criminals use modern technological devices to carry out criminal acts and to reduce the possibility of detection, they can hardly complain when the police take advantage of the inherent characteristics of those very devices to catch them,” the court said.

The Drug Enforcement Administration, which was running the case, acquired a court order — although not a search warrant — to monitor the suspected drug runner’s phone. But the court said neither may have been necessary for such cases. (One of the judges on the three-judge appellate panel said a warrant was needed.)

“We have two vague opinions and a lot of concern,” said Orin Kerr, a George Washington University law professor following the issue. “At some point in the next 10 years, the Supreme Court is probably going to have to figure out how it feels about cellphone location information.”

The uncertainty has prompted the Justice Department to tell law enforcement officials that, although they may not need search warrants when they seek GPS data, it would be wise to get them until the courts clarify the issue.

Legislation proposed by Sen. Ron Wyden (D-Ore.) and Rep. Jason Chaffetz (R-Utah) seeks to set the standards. “When the Supreme Court had an opportunity to settle the question, they all but begged the Congress to step in and provide clearer rules,” Wyden said.