After their arrests, federal investigators asked judges for access to what’s called “historical cell-tower location information,” or historical CSLI. The application wasn’t a warrant, which would have required investigators to show probable cause. Instead, they used what’s called a 2703(d) order, named after the provision of the federal Stored Communications Act that establishes it, to apply for the records.

After reviewing them, investigators in both cases determined that the suspects’ cellphones had interacted with cell towers near the sites of each robbery when the robberies occurred. Carpenter and Sanders were sentenced to 1,395 months—more than 116 years—in prison, while Graham received a 1,764-month sentence. The Fourth and Sixth Circuit Courts of Appeal upheld their respective convictions and the use of the historical CSLI data at trial, after which the defendants asked the Supreme Court for review.

At issue in both cases is the burden of proof police must meet before a judge can grant access to historical CSLI. The 2703(d) orders require a lower standard: Investigators must provide “specific and articulable facts showing that there are reasonable grounds to believe that the records or other information sought are relevant and material to an ongoing criminal investigation.” That’s considered to be a more relaxed threshold than what’s needed to obtain a typical warrant under the Fourth Amendment.

The burden-of-proof gap between 2703(d) orders and warrants springs from the third-party doctrine, a legal principle first articulated by the Supreme Court 40 years ago. Under the doctrine, Americans have no “reasonable expectation of privacy,” a key Fourth Amendment test, when voluntarily disclosing information to a third party like a phone company and or a bank. That gives police broad latitude to obtain a suspect’s information without a warrant when it’s held by a company or similar entity.

That leeway has drawn criticism from academics and civil-liberties advocates. Elizabeth Joh, a University of California, Davis, law professor, said the doctrine is one of two major Fourth Amendment issues that “really calls out for reconsideration.” (The other, she said, is the border-search exception, which lowers Fourth Amendment protections at or near ports of entry.) Both of the main Supreme Court decisions upon which the third-party doctrine is built date back to the 1970s, when telecommunications technology was far less advanced.

“The real issue is: Does it make sense to keep applying these pre-digital-era cases to a totally different context?” Joh asked.

At least one member of the Supreme Court has indicated a willingness to reevaluate the doctrine’s modern utility. In the 2012 case United States v. Jones, the justices unanimously ruled that attaching a GPS device to a suspect’s car and tracking them without a warrant violates the Fourth Amendment. Some justices, like the late Antonin Scalia, argued it constituted a trespass; others argued it violated a person’s reasonable expectations of privacy. But Justice Sonia Sotomayor went further and suggested revisiting the third-party doctrine in its entirety.