The Supreme Court on Monday put the brakes on the government’s use of high-tech monitoring devices to track motorists, ruling unanimously that police and the FBI violated the 4th Amendment by attaching a GPS device to a Jeep owned by a drug suspect.

The justices all agreed that the government needs a search warrant from a judge before it seeks to track a suspect by secretly installing a device on his car.

They were divided, however, as to what level of tracking would require a search warrant. Justice Antonin Scalia, speaking for a five-member majority, said the police erred because they attached the tiny device to the vehicle. He said the 4th Amendment was intended to protect against government searches on private property.

"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 said. “The government physically occupied private property for the purpose of obtaining information,” he said.

Such a search is unconstitutional unless officers obtained a search warrant from a judge. Chief Justice John G. Roberts Jr. and Justices Anthony Kennedy, Clarence Thomas and Sonia Sotomayor joined Scalia’s opinion.

Meanwhile, Justice Samuel A. Alito Jr. said he would go further and rule that the “long-term monitoring” of the vehicle with a tracking device violated the 4th Amendment regardless of whether the device was attached to a car. He took the view that the government violated a motorist’s right to privacy by tracking his movements for weeks on end.

Under Alito’s approach, police would need a search warrant for any use of a tracking device, whether or not it was attached to the car. Justices Ruth Bader Ginsburg, Stephen G. Breyer and Elena Kagan joined his opinion.

None of the justices agreed with the Justice Department’s view that the use of GPS device was a reasonable means of tracking a motorist on a public highway.

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-- David G. Savage in Washington

Photo: Justice Antonin Scalia. Credit: J. Scott Applewhite / Associated Press