Today’s cell phones enable people to stay connected to work, family and friends in ways that would have sounded like science fiction to past generations. But even some of the savviest of cell-phone users are unaware of the ways in which their devices may be used against them. To police and prosecutors, a cell-phone, which registers its location with the nearest cell phone tower every seven seconds, is like an individual tracking device. And, as the landscape of cell-phone towers becomes denser, the radius served by each tower becomes smaller, allowing police to track cell-phone users with greater and greater precision. In some areas served by “micro cell sites,” police are able to track cell-phone users within buildings, and even within individual floors and rooms within buildings.

Not surprisingly, law enforcement across the country have made increasing use of cell-phone tracking data to nab criminal suspects as well as to compile evidence in the cases against them — showing exactly where they were when the crime in question was committed. A recent study by the American Civil Liberties Union found that police use of cell-phone tracking data is so widespread that wireless companies now provide police departments with a menu of “surveillance fees” for tracking data and other types of information. Police typically are able to obtain such tracking data without a warrant, but this is where the New Jersey Supreme Court, in State v. Earls, drew the line.

The Earls case arose from a burglary investigation. In their search for the suspect and his girlfriend, whom the police believed was in danger, the police contacted T-Mobile, the suspect’s service provider, and obtained real-time information about the coordinates of the suspect’s cell phone. Based on the towers the suspect’s phone was in contact with, the police tracked him down at a motel, where they caught the suspect red-handed with a stolen flat-screen TV, jewelry and other fruits of the crime. After being charged with various crimes, the suspect-turned-defendant moved to suppress the evidence found at the motel based on the police’s failure to obtain a warrant before accessing his cell-phone tracking data. The trial court as well as the appellate court upheld the validity of the seizure. But the New Jersey Supreme Court reversed.

The New Jersey Supreme Court held that a person has a “reasonable expectation of privacy” — and thus a constitutional right to privacy — in their cell-phone location information. Clearly troubled by the level of intrusion allowed by modern cell-phone tracking technology, the court explained that “cell phones can now trace our daily movements and disclose not only where individuals are located at a point in time but also which shops, doctors, religious services and political events they go to, and with whom they choose to associate.” The court found that people who buy cell phones would not “reasonably expect them to be used by the government in that way.” As a result, held the court, the police are limited in their ability to access a person’s cell-phone tracking data in the same way that they are limited in their ability to search a person’s home: they must obtain a warrant from a magistrate supported by probable cause, unless some exception (such as a public emergency) justifies dispatching with the warrant requirement.

Because the police in Earls failed to obtain a warrant before obtaining the defendant’s cell-phone tracking data, the court held that their resulting search and seizure of the defendant and the evidence collected from the motel were subject to exclusion unless an exception applied. The court then remanded the case to the appellate court to determine whether one such exception — the “emergency aid” exception — applied based on the police’s fear that the suspect’s girlfriend was in danger.

The Earls decision, while important, is still just a small step toward providing greater privacy protection to cell-phone users. The court based its decision not on the Fourth Amendment but rather on a nearly identical provision in the New Jersey Constitution, which has been held to confer greater privacy rights than its federal analog. Still, the court reached its holding in part based on the recent United States Supreme Court case of United States v. Jones, 132 S. Ct. 945 (2012), in which the Court held that the warrantless installation of a GPS device on a suspect’s car violated the Fourth Amendment. Today’s cell-phones are essentially the same as GPS devices, the court reasoned, because they allow for continuous and precise monitoring of a person’s location.

Although the United States Supreme Court has not taken up the issue of cell-phone tracking data, the Earls case could prove to be persuasive to some justices should the issue come before them. The Earls decision will also undoubtedly be influential to other state courts considering whether their own state constitutions, or the Fourth Amendment, permit the police to obtain cell-phone tracking data without a warrant. The Ohio Supreme Court, for instance, has held that police must obtain a warrant before searching the contents of a person’s cell phone but has not yet addressed the issue of whether tower data is similarly protected. It is possible that state legislators (and, perhaps, members of Congress) may also look to Earls and similar decisions as support for joining states such as Montana in imposing statutory warrant requirements.