A federal appeals court ruled Wednesday that a probable-cause warrant under the Fourth Amendment is required for the police to obtain a suspect's cell-site data.

Further Reading Cops shouldn’t have easy access to 220+ days of cell location data, lawyers say

The decision by the Fourth US Circuit Court of Appeals gives the Supreme Court, which has never ruled on the issue, ammunition to resolve a modern-day privacy controversy affecting the tens of millions of American mobile phone users. Until Wednesday, all the federal appellate courts that have decided the issue have ruled for the government's proposition that cell-site records are not constitutionally protected.

But the Richmond, Virginia-based appeals court didn't buy the government's assertion that cell-site records are business records investigators may obtain from the telcos by asserting that there are reasonable grounds to believe the data is relevant to an investigation. The government's argument is known in legal jargon as the third-party doctrine. That's a much lower standard than the constitutional, probable-cause standard, according to the appellate court's 2-1 ruling: (PDF)

As we have explained, society recognizes an individual’s privacy interest in her movements over an extended time period as well as her movements in private spaces. The fact ￼￼￼￼￼￼￼￼￼that a provider captures this information in its account records, without the subscriber’s involvement, does not extinguish the subscriber’s reasonable expectation of privacy. Applying the third-party doctrine in this context would simply permit the government to convert an individual’s cell phone into a tracking device by examining the massive bank of location information retained by her service provider, and to do so without probable cause.

The case the appeals court decided concerned Aaron Graham and Eric Jordan, convicted of being involved in several Baltimore-area heists. The duo had been arrested regarding one robbery. But a seven-month historical examination into their mobile phone calling and texting records linked them to the location of where other stores had been robbed. The court, led by President Obama appointee Judge Andre Davis, explained that knowing the location of the mobile phone towers where calls and texts were made can track one's whereabouts.

A cell phone connects to a service provider’s cellular network through communications with cell sites, occurring whenever a call or text message is sent or received by the phone. The phone will connect to the cell site with which it shares the strongest signal, which is typically the nearest cell site. The connecting cell site can change over the course of a single call as the phone travels through the coverage area.

In any case, the appeals court declined to suppress the evidence because the court found that the "government relied in good faith" that the law did not require warrants. The court's decision, however, provides notice to the authorities in North Carolina, Maryland, South Carolina, and Virginia that warrants are now required. Those are the states that the appeals court covers.

In dissent, Judge Diana Gribbon Motz, a President Clinton appointee, wrote that the majority's ruling "flies in the face of the Supreme Court's well-established third-party doctrine."

Wednesday's decision comes a week after the American Civil Liberties Union asked the Supreme Court to resolve the issue of whether warrants were required for the cell-site data. That case concerned a Florida man sentenced to life for a string of robberies in a case built with two months' worth of cell-site data.