The Supreme Court ruled Friday in Carpenter v. United States that law enforcement needs a warrant in order to track a person's location information from cellphone towers over an extended period of time. Chief Justice John Roberts sided with the liberals in the 5-4 decision, writing in the opinion that people "compulsively carry cellphones with them at all times," making monitoring that data "near perfect surveillance" akin to attaching "an ankle monitor to the phone's user." The government had argued unsuccessfully that "cellphone users voluntarily reveal to their providers information about their proximity to cell towers," so customers "cannot reasonably expect" providers to conceal that information from the government.

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