There was enormously important Fourth Amendment news from the Supreme Court on Monday: The justices agreed to review the U.S. Court of Appeals for the 6th Circuit’s decision in Carpenter v. United States, one of the long-pending cases on whether the Fourth Amendment protects government access to historical cell-site records.

This is a momentous development, I think. It’s not an exaggeration to say that the future of surveillance law hinges on how the Supreme Court rules in the case. Let me say a bit about the case, the issues it will decide and why it matters.

I. The Facts of the Case

Carpenter involves a string of armed robberies that occurred over a two-year period. A group of men (at least five of them) would go into cellphone stores armed with guns, order the customers and employees to the back, and steal the phones. Carpenter was the lead organizer of the conspiracy, and he often supplied the guns, acted as a lookout and would signal when each robbery was to begin.

One of Carpenter’s conspirators confessed to the crime and gave the government his cellphone number and the numbers of the other conspirators (16 numbers total). The government applied for three different court orders for the cell-site records associated with those numbers, which included Carpenter’s number. Specifically, the orders sought “cell site information” for Carpenter’s phone “at call origination and at call termination for incoming and outgoing calls.” The government obtained the orders under the Stored Communications Act. They complied with the statute, but the statute requires only reasonable suspicion and not probable cause.

The order that covered Carpenter was directed at his cellphone provider MetroPCS. MetroPCS produced 127 days of historical cell-site records. (Sprint produced another seven days of historical cell-site records for Carpenter’s phone from a time window when he was “roaming” and Sprint picked up his service instead of MetroPCS.) Together with the orders obtained, the records showed that that the phones of the alleged conspirators were within distances ranging from a half-mile up to two miles of the robberies at the time they occurred. Specifically, Carpenter’s phone was shown to be in communication with cell towers near four robberies over a five-month window.

II. The Legal Issues

Here is how counsel for the petitioner framed the “question presented”:

Whether the warrantless seizure and search of historical cell phone records revealing the location and movements of a cell phone user over the course of 127 days is permitted by the Fourth Amendment.

And here’s how the United States redrafted the question presented in its brief in opposition:

Whether the government’s acquisition, pursuant to a court order issued under 18 U.S.C. 2703(d), of historical cell-site records created and maintained by a cellular-service provider violates the Fourth Amendment rights of the individual customer to whom the records pertain.

I gather, then, that the case will consider two distinct questions. First, is the collection of the records a Fourth Amendment search? And second, if it is a search, is it a search that requires a warrant?

Notably, neither side sought review of whether the good-faith exception applies if the answer to both of these questions is “yes.” The parties are asking only for a ruling on the merits, with any remedies decision bifurcated for review on remand if the Supreme Court reverses.

III. Why The Case Matters

The Carpenter case is tremendously important, I think. The structure of modern surveillance law is built on the idea that the contents of communications receive Fourth Amendment protection but that non-content metadata — records about communications, and other third-party business records — do not. That has been the rule since the 19th century for postal letters, and it has been the rule since 1979 for phone calls. Carpenter will help determine if that basic rule framework will remain, or if the Supreme Court will amend it somewhat or even dramatically change it.

Part of the importance of the case is that it’s not just about cell-site records. Although the case is formally about cell-site records, it’s really about where to draw lines in terms of what network surveillance triggers the Fourth Amendment and how the Fourth Amendment applies. The justices can’t answer how the Fourth Amendment applies to cell-site records without providing a framework for how the Fourth Amendment applies to many other forms of surveillance, such as visual surveillance, obtaining traditional phone records, obtaining e-mail transactional records, obtaining credit card records and the like.

For example, readers will recall the debate over the mosaic theory of the Fourth Amendment. Among the issues likely to be pressed in Carpenter is whether the justices should adopt or reject the mosaic theory. Note that the question presented focuses on the fact that the records covered 172 days. Should the length covered by the records matter? Is evidence collection for a short time window no search that becomes a search because the records spanned a long time window?

Plus, remember that the justices will have two questions: what a search is, and when searches are reasonable. Most will focus on the first question, but note that the two issues go together. As I explained here, the broader the court interprets “search,” the more pressure there is to water down reasonableness. The narrower the definition of “search,” the stronger the reasonableness standard tends to be. This creates some interesting dynamics. For example, you might get a ruling that there is no search but that retains the traditional default warrant rule for searches. On the other hand, you might get a ruling that a search occurred but that authorizes a new category of warrantless surveillance. This is just speculation, of course, but I suspect the briefing will urge major doctrinal innovations on both questions.

IV. Why Did the Justices Take the Case?

Some will speculate that the Supreme Court would have taken the case only if it were going to reverse. I have no idea how the court will rule, but I tend to doubt that. If I had to guess, I would guess that the court took these cases because they’re really important. The lower court rulings are based on the third-party doctrine, and none of the current justices were on the court the last time the justices decided a case on the third-party doctrine. It’s pretty sensible to have the current Supreme Court weigh in.

As it happens, I think the third-party doctrine is essential to technological surveillance in a digital age. As I see it, the doctrine is needed to maintain the essential balance on which Fourth Amendment law has been built and on which it evolves in response to new technology. Prominent alternatives, like the mosaic theory, strike me as a dead end. But it makes a lot of sense for the justices to review these cases and decide whether they agree — and if not, identify what new framework should replace it.

V. Lots of Blogging Ahead

Finally, I’ll probably be doing a lot of carpentry (that is, blogging about the issues raised in Carpenter) over the next few months. A lot of my academic work in the past decade has been about issues that touch on the case, so it will be really fun to see what the justices do.