One of the biggest cases for the US Supreme Court’s current term could mark a watershed moment for the Fourth Amendment. In Carpenter v. United States, the court will consider whether police need probable cause to get a search warrant to access cell site location information (CSLI), data that's automatically generated whenever a mobile phone connects to a cell tower. Not only does this case offer a chance to protect privacy rights for cell phones, Carpenter also provides an opportunity to reevaluate an antiquated legal theory, called the third-party doctrine, that underpins many government surveillance programs.

WIRED OPINION ABOUT Nick Sibilla (@nick_sibilla) is a writer and legislative analyst at the Institute for Justice, a libertarian-leaning public interest law firm.

Once considered a luxury, cell phones are now a modern essential, with 95 percent of Americans owning a mobile phone. With the rise in smartphone ownership, the amount of data carried over these networks has skyrocketed by more than 3,500 percent in just the past seven years. To handle that surge, there are at least 300,000 cell sites in operation nationwide, according to the Electronic Frontier Foundation. Phones connect to those towers as frequently as every 7 seconds.

Given this pervasiveness, it should come as no surprise that CSLI grants law enforcement enormous power to monitor Americans’ movements. Last year, AT&T and Verizon together received nearly 125,000 requests from law enforcement to access CSLI for criminal investigations.

Timothy Carpenter found that out first-hand. After Carpenter was identified as a suspect for organizing a series of armed robberies in the Detroit area, the FBI retrieved months of his historical CSLI without a warrant. Thanks to those call-detail records, his phone was placed within a half-mile to 2 miles of the robberies, and at the times when they occurred. Carpenter was ultimately convicted and sentenced to 116 years in prison.

Carpenter, who is represented by the American Civil Liberties Union, argued that accessing his CSLI without a proper warrant violated his Fourth Amendment rights protecting against unreasonable searches and seizures. But the Sixth Circuit ruled against him. The court held that CSLI is covered by the third-party doctrine, which states that information customers voluntarily provide to a third party (like a telecom company or a bank) is viewed as outside the protection of the Fourth Amendment. Carpenter appealed that decision, and his case is now before the Supreme Court.

The aftermath of the Supreme Court's decision to apply the third-party doctrine to bank records provides a chilling case study. The modern third-party doctrine emerged in the 1976 case of Mitchell Miller, a bootlegger who was convicted after his bank records were subpoenaed—without a warrant—under the Bank Secrecy Act. Miller claimed that the warrantless subpoenas violated his Fourth Amendment right to be free from unreasonable searches and seizures. As such, he argued that his bank documents were illegally seized and should be suppressed as evidence.

But in its decision United States v. Miller, the Supreme Court ruled that Miller had no “legitimate ‘expectation of privacy,” because in the court’s view, “the Fourth Amendment does not prohibit the obtaining of information revealed to a third party and conveyed by him to Government authorities.” Three years later, in Smith v. Maryland, the Supreme Court relied on Miller to extend the third-party doctrine to telephone numbers recorded by phone companies.

The fallout from those decisions has been dire. Enabled by the third-party doctrine, courts have allowed the federal government to create massive surveillance dragnets that routinely mine records from millions of Americans.

Consider the Bank Secrecy Act, whose recordkeeping requirements ultimately led to Miller’s conviction. Today, the reports generated by the Act have spawned a surveillance behemoth. In 2015, the Treasury Department’s Financial Crimes Enforcement Network boasted that the Act’s data “includes nearly 190 million records,” with roughly 30,000 data searches happening every day. To ensure what the government calls "the broadest possible use,” FinCEN has granted more than 10,000 agents and analysts from 350 different government agencies direct access to those reports, which allows those individuals to review records without a warrant or a subpoena. By failing to keep up with clear advances in technology, the judiciary has exempted a treasure trove of data from any constitutional scrutiny.

The third-party doctrine provides the means to surveil, and the nation’s civil forfeiture laws offer a powerful motive. Under federal law, law enforcement agencies can forfeit property without ever filing criminal charges. And they can even keep what they confiscate, creating a perverse incentive to pursue more forfeiture cases.