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In its next term, the US Supreme Court will take up a landmark case concerning law enforcement’s ability to track cellphones without a warrant.

As it stands, police in some states are able to acquire a person’s cellphone location history without demonstrating probable cause, and they do so tens of thousands of times each year. But due to the amount of private information contained in these historical records—you could tell, for example, if a person visited a therapist or an abortion clinic, or with whom they spent the night—groups such as the American Civil Liberties Union (ACLU) argue that, without a warrant, this constitutes an invasive search and violates a person’s Fourth Amendment rights.

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Sometime after October, SCOTUS will hear arguments in the case of Carpenter v. United States, to which the ACLU is co-counsel. Timothy Carpenter and his accomplices were convicted in 2014 of robbing a string of cellphone stores. Due to his prior convictions for dealing cocaine, and because five of the six robberies were armed, Carpenter received a particularly harsh, 119-year prison sentence.

But whether Carpenter is guilty or not is not the issue. Police obtained months’ worth of cellphone records after the 29-year-old was arrested revealing nearly 13,000 separate points of location data. The records were acquired under a provision of the federal Stored Communications Act, which means that investigators were not required to demonstrate probable cause that a crime was committed—instead, it was simply explained to a judge that the records were likely to show Carpenter’s location at the time of the robberies.

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And in this case, the police were right: The historical cellphone records proved that Carpenter’s phone had interacted with the base receiver stations (aka “cell towers”) nearest the stores as they were being robbed. In a 2-1 decision, the Sixth Circuit Court of Appeals upheld the lower court ruling, finding that a warrant was not required for the location data which led to Carpenter’s conviction.

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At the heart of this case is what’s called “third-party doctrine,” a legal theory that came into being in the 1970s and states that a person has “no legitimate expectation of privacy” in information voluntarily turned over to third parties. In other words, if you freely surrender otherwise private information it is no longer protected by the Fourth Amendment.

In 2012, a Maryland district court held that historical cell phone location history falls under the third-party doctrine because a cellphone owner volunteers that information to the cellphone company. But legal scholars have argued that, instead, this merely demonstrates that the 40-year-old legal theory is inconsistent with the way private information is handled in the 21st century.

“Because cellphone location records can reveal countless private details of our lives, police should only be able to access them by getting a warrant based on probable cause,” Nathan Wessler, a staff attorney with the ACLU Speech, Privacy, and Technology Project, said in a statement. “The time has come for the Supreme Court to make clear that the longstanding protections of the Fourth Amendment apply with undiminished force to these kinds of sensitive digital records.”

While SCOTUS has not previously weighed in on this topic, some states, such as California and Montana, have taken unilateral action to impose privacy protections on historical cell-site data. This has created a patchwork of varying rules across the country.

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In Illinois, for instance, a warrant is required for real-time tracking, but not for access to historical data, while in Texas, no warrant is required for location data of any kind. And in Florida, Maryland, and Virginia, there have been conflicting laws and opinions on the matter, making this particular SCOTUS case one of particular interest to digital rights groups and law enforcement alike.