In the past five years, the extent and specificity of this data has likely increased. Smartphones check in much more often than plainer feature phones, communicating with the carrier—and thus registering location data—whenever they receive a push notification or download something in the background.

Right now, U.S. law enforcement at any level can ask mobile carriers for that database of information for a customer or customers and receive it, virtually as far back in time as they need. If they want a week of data, they can get it; if they want seven months, they can just ask for that, too. Even a county-level detective, wanting to get at that information, never has to prove probable cause to a judge. (Probable cause is the legal standard for granting a warrant in the U.S.: It’s what law enforcement has to prove before they can search someone’s home.)

Instead, law enforcement has to prove to a much lower legal standard, called “reasonable suspicion.” This entails only that “specific and articulable facts” would lead the government to infer suspicion of someone in a crime: It’s the same legal standard cops use to judge whether someone on the street can be frisked for weapons. (They would need to meet probable cause to search that same person for drugs.)

Reasonable suspicion can often be quickly indicated to a judge, who can then issue a court order under the Electronic Communications Privacy Act. In 2012, all major U.S. carriers indicated to ProPublica that they comply with those orders when served.

Over the past few years, two different U.S. appeals courts have ruled on this practice. Both courts, the Fifth and Eleventh Circuits, said that it was perfectly legal: that the process of gathering cell-location data without a warrant didn’t violate the Fourth Amendment’s protection against “unreasonable searches and seizures” of homes, persons, and effects, without a warrant issued under probable cause. That’s because both were working off a piece of 1970s-era precedent called the “third-party doctrine,” which holds that people do not have a reasonable expectation of privacy for information voluntarily given to a third party, like a bank or telephone company.

This week, the Fourth Circuit disagreed. Ruling in Graham v. U.S., it said that ​gathering such information without a warrant violates the “reasonable expectation of privacy” to which Americans are entitled.

“The Fourth Circuit held, we think correctly, that those old cases from the 1970s were about the voluntary conveyance of really limited, discrete pieces of information to a phone company, or to a bank, or some similar business,” said Nathan Wessler, a staff attorney at the American Civil Liberties Union who filed briefs in the case. “When we’re talking about these cellphone location records, people are not voluntarily conveying that information to their cellphone service providers at all.”