The Supreme Court said Monday that law enforcement authorities might need a probable-cause warrant from a judge to affix a GPS device to a vehicle and monitor its every move – but the justices did not say that a warrant was needed in all cases.

The convoluted decision (.pdf) in what is arguably the biggest Fourth Amendment case in the computer age, rejected the Obama administration's position that attaching a GPS device to a vehicle was not a search. The government had told the high court that it could even affix GPS devices on the vehicles of all members of the Supreme Court, without a warrant.

"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,'" Justice Antonin Scalia wrote for the five-justice majority. The majority declined to say whether that search was unreasonable and required a warrant.

All nine justices, however, agreed to toss out the life sentence of a District of Columbia drug dealer who was the subject of a warrantless, 28-day surveillance via GPS.

Four justices in a minority opinion said that the prolonged GPS surveillance in this case amounted to a search needing a warrant. But the minority opinion was silent on whether GPS monitoring for shorter periods would require one.

Justice Sonia Sotomayor voted with the majority, but wrote in a separate, solo opinion that both the majority and minority opinions were valid. She also suggested that Americans have more rights to privacy in data held by phone and internet companies than the Supreme Court has held in the past.

"I think it's fair to say, the use of a a GPS device like this requires a warrant where they are tracking him for a long time," Thomas Goldstein, who has argued dozens of cases before the Supreme Court, said in a telephone interview.

The Justice Department maintained it had probable cause in the case, though not a valid warrant. The majority said because of procedural rules, it would not decide whether the "search" in this case required a warrant. "We consider that argument forfeited," the majority wrote.

The Justice Department declined to comment.

Walter Dellinger, the lawyer for the drug dealer who appealed his conviction, said the decision, no matter how disjointed, means "that almost any use of GPS electronic surveillance of a citizen's movement will be legally questionable unless a warrant is obtained in advance."

The justices agreed to hear the case in a bid to settle conflicting lower-court decisions — some of which ruled a warrant was necessary, while others found the government had unchecked GPS surveillance powers. For the moment, the conflict is unresolved and "will take more lawsuits," Orin Kerr, a Fourth Amendment scholar and former Justice Department prosecutor, said in a telephone interview.

One of the Obama administration's main arguments in support of warrantless GPS tracking was the high court's 1983 decision in United States v. Knotts, in which the justices ruled it was OK for the government to use beepers known as "bird dogs" to track a suspect's vehicle without a warrant. In that case, the police had the consent of that truck's owner, which was not the case in the opinion decided Monday, Scalia wrote.

In the Supreme Court case decided Monday, the U.S. Court of Appeals for the District of Columbia Circuit had ruled that the Fourth Amendment rights of suspected District of Columbia drug dealer Antoine Jones had been violated by the month-long warrantless attachment of a GPS underneath his car. The lower court had reversed Jones' conviction, saying the FBI needed a warrant to track Jones.

Scalia's majority opinion, which was joined by Chief Justice John Roberts, and Justices Anthony Kennedy, Clarence Thomas and Sonia Sotomayor, said placing the device on the suspect's car amounted to a search.

In a separate opinion, written by Justice Samuel Alito, and joined by Justices Ruth Bader Ginsburg, Stephen Breyer and Elena Kagan, Alito wrote that Scalia's opinion was "unwise" and said it should have examined "whether respondent's reasonable expectations of privacy were violated by the long-term monitoring of the movements of the vehicle he drove."

"For these reasons, I conclude that the lengthy monitoring that occurred in this case constituted a search under the Fourth Amendment," Alito wrote. He wrote that the police – to avoid any ambiguity on how long the monitoring must take place to demand a warrant – "may always seek a warrant" to avoid committing wrongdoing.

Justice Sotomayor also wrote separately that "the government usurped Jones' property for the purpose of conducting surveillance on him, thereby invading privacy interests long afforded and undoubtedly entitled to, Fourth Amendment protection."

During oral arguments in the case in November, a number of justices invoked the specter of Big Brother if the police could secretly attach GPS devices on Americans' cars without getting a probable-cause warrant.

The last time the high court considered the Fourth Amendment, technology and privacy in a big-ticket case was a decade ago, when the justices ruled that the authorities must obtain search warrants to employ thermal-imaging devices to detect indoor marijuana-growing operations, saying the imaging devices carry the potential to "shrink the realm of guaranteed privacy."

The Obama administration urged the court to reinstate the conviction and life sentence of Jones, a suspected cocaine dealer whose vehicle was tracked via GPS for a month without a court warrant.

The government told the justices during oral arguments that that GPS devices have become a common tool in crime fighting, saying it is employed "thousands" of times annually.

(This post was updated to reflect that GPS monitoring does not always require a warrant.)