Crossposted from the NY Times

WASHINGTON — The Supreme Court on Monday unanimously ruledthat 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 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.

Walter Dellinger, a lawyer for the defendant in the case and a former acting United States 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 G.P.S. electronic surveillance of a citizen’s movement will be legally questionable unless a warrant is obtained in advance.”

Though 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 Monday, United States v. Jones, No. 10-1259, 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 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.

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