WASHINGTON — The Supreme Court on Monday unanimously ruled that the police violated the Constitution when they placed a Global Positioning System tracking device on a suspect’s car and tracked its movements for 28 days.

But the justices divided 5-to-4 on the rationale for the decision, with the majority saying that the problem was the placement of the device on private property. That ruling avoided many difficult questions, including how to treat information gathered from devices installed by the manufacturer and how to treat information held by third parties like cellphone companies.

The case concerned Antoine Jones, who was the owner of a Washington nightclub when the police came to suspect him of being part of a cocaine-selling operation. They placed a tracking device on his Jeep Grand Cherokee without a valid warrant, tracked his movement for a month and used the evidence they gathered to convict him of conspiring to sell cocaine. He was sentenced to life in prison.

The United States Court of Appeals for the District of Columbia Circuit overturned his conviction, saying the sheer amount of information that had been collected violated the Fourth Amendment, which bars unreasonable searches.

The Supreme Court affirmed that decision, but on a different ground. “We hold that the government’s installation of a G.P.S. 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 majority. Chief Justice John G. Roberts Jr. and Justices Anthony M. Kennedy, Clarence Thomas and Sonia Sotomayor joined the majority opinion.

“It is important to be clear about what occurred in this case,” Justice Scalia went on. “The government physically occupied private property for the purpose of obtaining information. We have no doubt that such a physical intrusion would have been considered a ‘search’ within the meaning of the Fourth Amendment when it was adopted.”

In a concurrence for four justices, Justice Samuel A. Alito Jr. faulted the majority for trying to apply 18th-century legal concepts to 21st-century technologies. What should matter, he said, is the contemporary reasonable expectation of privacy.

“The use of longer term G.P.S. monitoring in investigations of most offenses,” he wrote, “impinges on expectations of privacy.” Justices Ruth Bader Ginsburg, Stephen G. Breyer and Elena Kagan joined the concurrence.

“We need not identify with precision the point at which the tracking of this vehicle became a search, for the line was surely crossed before the 4-week mark,” Justice Alito wrote. “Other cases may present more difficult questions.”

All of the opinions appeared to acknowledge that changes in technology are fast outpacing the law’s ability to address them.

In the concurrence, for instance, Justice Sotomayor wrote that “it may be necessary to reconsider the premise that an individual has no reasonable expectation of privacy in information voluntarily disclosed to third parties.”

“People disclose the phone numbers that they dial or text to their cellular providers; the URLs that they visit and the e-mail addresses with which they correspond to their Internet service providers; and the books, groceries, and medications they purchase to online retailers,” she wrote. “I for one doubt that people would accept without complaint the warrantless disclosure to the government of a list of every Web site they had visited in the last week, or month, or year.”