The Supreme Court on Monday unanimously restricted the police’s ability to use a GPS device to track criminal suspects in a first test of how privacy rights will be protected in the digital age.

The court rejected the government’s view that long-term surveillance of a suspect by GPS tracking is no different than traditional, low-tech forms of monitoring. But its decision was nuanced and incremental, leaving open the larger questions of how government may use the information generated by modern technology for surveillance purposes.

Still, the decision reversing the conviction of suspected D.C. drug kingpin Antoine Jones was a “landmark ruling in applying the Fourth Amendment’s protections to advances in surveillance technology,” said Washington lawyer Andrew Pincus, who filed a brief on Jones’s behalf.

The court without dissent agreed that prosecutors violated Jones’s rights when they attached a GPS device to his Jeep and monitored his movements for 28 days. In one of the Washington region’s most celebrated drug trials, the nightclub owner was convicted and sentenced to life in prison.

But while all the justices agreed with that outcome, they split 5 to 4 in their reasoning.

The Supreme Court building in Washington. (Andrew Harrer/Bloomberg)

Justice Antonin Scalia wrote for the majority that it was the attachment of the device that violated the Fourth Amendment’s protection against unreasonable searches and seizures.

“We hold that the government’s installation of a GPS device on a target’s vehicle, and its use of that device to monitor the vehicle’s movements, constitutes a ‘search,’ ” Scalia wrote.

Scalia did not hold that a warrant was always necessary. But Walter Dellinger, who helped represent Jones at the Supreme Court, said the decision means that any use of GPS technology by law enforcement without a warrant “would be a risky undertaking.”

Scalia’s limited ruling was joined by Chief Justice John G. Roberts Jr. and Justices Anthony M. Kennedy, Clarence Thomas and Sonia Sotomayor. Scalia said that electronic surveillance, if achieved without having to physically trespass on a person’s property, still may be “an unconstitutional invasion of privacy.”

He said there was “no reason for rushing forward” to resolve more complicated issues than those presented in Jones’s case.

But it was those difficult questions — about society’s expectation of privacy in an increasingly technological world — that had animated the court’s consideration of the case. In an intense hour-long oral argument in November, the Big Brother of George Orwell’s novel “1984” was referenced six times.

In separate opinions, Sotomayor and Justice Samuel A. Alito Jr. wrote of the sweeping changes technology has brought to society that do not involve government intrusions.

“In the course of carrying out mundane tasks,” Sotomayor wrote, Americans disclose the phone numbers they dial, the URLs they visit, “the books, groceries and medications they purchase.”

Alito wrote of toll booths that record a motorist’s travels, cars that come ready to broadcast their locations and 322 million wireless devices in use nationally.

Alito said that the court should have more clearly addressed some of those innovations instead of deciding a case about a “21st-century surveillance technique” by using “18th-century tort law.”

“The court’s reasoning largely disregards what is really important (the use of a GPS for the purpose of long-term tracking) and instead attaches great significance to something that most would view as relatively minor (attaching to the bottom of a car a small, light object that does not interfere in any way with the car’s operation),” Alito wrote.

Alito’s point was that “the use of longer term GPS monitoring in investigations of most offenses impinges on expectations of privacy.”

“For such offenses,” he wrote, “society’s expectation has been that law enforcement agents and others would not — and indeed, in the main, simply could not — secretly monitor and catalogue every single movement of an individual’s car for a very long period.”

He was joined by Justices Ruth Bader Ginsburg, Stephen G. Breyer and Elena Kagan.

The key to the court’s more narrow decision on the case seemed to be Sotomayor. She praised Alito’s “incisively” written concurrence and expressed the most concern about the loss of personal privacy. But she said the government’s intrusion on Jones’s property was enough to decide the case, and “resolution of these difficult questions” could wait.

The Supreme Court’s decision Monday upheld a panel of the U.S. Court of Appeals for the D. C. Circuit that said Jones’s conviction must be reversed — although it, like Alito, was concerned about the length of the surveillance. Jones remains imprisoned while prosecutors decide whether to retry him.

Law enforcement could have avoided the issue in 2005 if officers had more closely followed the instructions of a judge who issued a warrant authorizing the use of the GPS device. The judge said it had to be installed within 10 days while Jones’s Jeep was in Washington. Instead, it was installed after 11 days, while the vehicle was in Maryland.

“The U.S. Attorney’s Office for the District of Columbia is carefully reviewing the court’s opinion and evaluating its options,” a spokesman said in a statement. “Beyond that, the office has no comment at this time.”

A. Eduardo Balarezo, who represented Jones in his initial case and his retrial, said the GPS evidence was “highly critical” to the government’s case because it linked Jones to a Fort Washington stash house, where police and FBI agents found nearly $1 million in cash almost 100 kilos of cocaine.

The case is United States v. Jones .

Staff writer Keith Alexander contributed to this report.