While you cruise around, checking Twitter on your phone, texting your friends about dinner plans, or, I dunno, using your cell phone for an actual phone call, your phone is connecting to nearby cell towers, always searching for the best signal. Cellular signal providers, and (by request) police, can therefore get a good handle on your whereabouts, based on the towers your cell phone is "pinging." And even though this might not seem like particularly invasive data -- after all, you're probably updated your Facebook status with your location added -- it can provide law enforcement with a comprehensive view of your daily life.
The case involved Timothy Carpenter, the supposed architect of a series of armed robberies of electronics stores in the Detroit area. Carpenter allegedly planned the robberies, provided weapons, and served as a lookout in a stolen car across the street from the targeted stores. Police and prosecutors got access to cell-site tower data covering over 127 days and 12,898 locations. Many of those locations included robbery sites, and Carpenter was convicted and given a 116-year prison sentence.
But, as the Supreme Court noted, police also gained insight into Carpenter's whereabouts for long stretches before and after the robberies, including his sleeping and churchgoing habits. "Mapping a cell phone's location over the course of 127 days provides an all-encompassing record of the holder's whereabouts," wrote Chief Justice John Roberts, for the 5-4 majority. "As with GPS information, the timestamped data provides an intimate window into a person's life, revealing not only his particular movements, but through them his 'familial, political, professional, religious, and sexual associations.'" Therefore, if police wanted access to all that information, they would need a warrant first.
The Court pointed to two factors distinguishing Carpenter's case from previous decisions that had been more permissive to warrantless cell phone data searches and demonstrate that cell phone users aren't sharing information about their location with their carriers voluntarily. First, "seismic shifts in digital technology" allow wireless carriers to gather "deeply revealing" information about cell phone owners. Second, the Court conceded "that carrying one is indispensable to participation in modern society." Therefore, cell-site data implicates the Fourth Amendments protections against unreasonable search and seizure, and law enforcement generally will need to obtain a warrant to acquire location records.
The Court conceded a few exceptions to that rule, however. Police may be able to obtain cell-site location records without a warrant if:
They are gathering records in real time;
They are requesting records for a shorter amount of time than in Carpenter's case (around four months);
They are gathering data in response to emergencies like "bomb threats, active shootings, and child abductions"; or
They are getting information about all the phones connected to a particular tower at a particular time.
But if police are asking cell companies for location data on a particular phone for an extended amount of time, they'll need a warrant. If you have questions about cell phone data in your criminal case, consult an experienced criminal defense attorney.