WASHINGTON - The Supreme Court unanimously ruled yesterday that the police violated the Constitution when they placed a global positioning system tracking device on a suspect’s car and monitored its movements for 28 days.

But the justices were 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, such as cellphone companies.

Walter Dellinger, a lawyer for the defendant in the case and a former acting US solicitor general, said the decision “is a signal event in Fourth Amendment history.’’

“Law enforcement is now on notice,’’ he said, “that almost any use of GPS electronic surveillance of a citizen’s movement will be legally questionable unless a warrant is obtained in advance.’’

Although the ruling was limited to physical intrusions, the opinions in the case collectively suggested that a majority of the justices are prepared to apply broad Fourth Amendment privacy principles unrelated to such intrusions to an array of modern technologies, including video surveillance in public places, automatic toll collection systems on highways, devices that allow motorists to signal for roadside assistance, and records kept by online merchants.

The case decided yesterday 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 vehicle without a warrant, tracked his movements 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 US 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 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 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,’’ 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 GPS 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 four-week mark,’’ Alito wrote. “Other cases may present more difficult questions.’’

In other action yesterday:

■ The high court said it would not hear arguments from a conservative watchdog group that wants Kagan disqualified from deciding the constitutionality of President Obama’s national health care overhaul. Freedom Watch asked the high court for time to demand disqualification of Kagan during arguments in the case because she was solicitor general under Obama.

■ The justices unanimously struck down a California law that had sought to keep out of the food supply animals that are unable to walk. The justices said the state law conflicted with the federal Meat Inspection Act, which already requires the immediate euthanasia of “downer’’ animals and bars their slaughter or sale.

■ The court said rules requiring sex offenders to update their registration when crossing state lines do not automatically apply to those who committed their crimes before the law was passed. The court ruled, 7 to 2, that Billy Joe Reynolds can challenge his arrest for violating the Sex Offender Registration and Notification Act after moving from Missouri to Pennsylvania.