The NCSL Blog


By Lisa Soronen

In a long-awaited decision in Carpenter v. United States, the U.S. Supreme Court held 5-4 that the Fourth Amendment requires the government to receive a warrant to obtain cell-site location information (CSLI).

Cell towerChief Justice John Roberts, writing the majority opinion, provides an explanation of how CSLI works.

“Cell phones continuously scan their environment looking for the best signal, which generally comes from the closest cell site. Most modern devices, such as smartphones, tap into the wireless network several times a minute whenever their signal is on, even if the owner is not using one of the phone’s features. Each time the phone connects to a cell site, it generates a time-stamped record known as cell-site location information (CSLI).”

Robbery suspects gave the FBI Timothy Carpenter’s name and cell phone number as an accomplice who participated in a number of robberies with them. Prosecutors obtained Carpenter’s CSLI for over 100 days and were able to show that Carpenter was located at four of the robberies at the exact time they occurred. Under the Stored Communications Act (SCA), prosecutors applied for a less-stringent court order rather than a warrant to obtain the records. 

In concluding that obtaining CSLI was a search, the court rejected the argument that the “third-party” doctrine applies in this case. In previous cases the court has held that people have no legitimate expectation of privacy in information voluntarily turned over to third parties, meaning such information isn’t protected by the Fourth Amendment.

According to the court, the information to which the court applied the third-party doctrine in previous cases (bank records and dialed phone numbers) isn’t comparable to “the ability to chronicle a person’s past movements through the record of his cell phone signals.”

The court further explained:  “With access to CSLI, the government can now travel back in time to retrace a person’s whereabouts, subject only to the retention polices of the wireless carriers, which currently maintain records for up to five years. Critically, because location information is continually logged for all of the 400 million devices in the United States—not just those belonging to persons who might happen to come under investigation— this newfound tracking capacity runs against everyone. [P]olice need not even know in advance whether they want to follow a particular individual, or when.”

After deciding that obtaining CSLI is a Fourth Amendment search, the court concluded the government needs to obtain a warrant because warrants are typically required where “a search is undertaken by law enforcement officials to discover evidence of criminal wrongdoing.”

The SCA’s requirement to show “reasonable grounds” for believing that the records were “relevant and material to an ongoing investigation ... falls well short of the probable cause required for a warrant.”   

In a dissenting opinion, Justice Anthony Kennedy disagreed with the court’s analysis that tracking CSLI constitutes a search, arguing that cell phone users have no reasonable expectation of privacy in records they do not “own, possess, control, or use.” Kennedy was also critical of the effects of the majority’s new rule, calling it “an unprincipled and unworkable line” that would “frustrate principled application of the Fourth Amendment in many routine yet vital law enforcement operations.”

Justice Samuel Alito agreed, adding in his own dissenting opinion that the court’s decision threatened to undermine the government’s power to subpoena by holding its request for location data to too high of a standard. Finally, Justice Neal Gorsuch notably suggested in a dissenting opinion that state legislatures should have more of a role to play in establishing property rights in cellphone data, allowing users to claim more traditional Fourth Amendment protections.

Lisa Soronen is executive director of the State and Local Legal Center and a frequent contributor to the NCSL Blog on judicial issues.


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This blog offers updates on the National Conference of State Legislatures' research and training, the latest on federalism and the state legislative institution, and posts about state legislators and legislative staff. The blog is edited by NCSL staff and written primarily by NCSL's experts on public policy and the state legislative institution.