A criminal case examining the Fourth Amendment implications of cell-site simulators, also known as stingrays, has finally reached the 7th Circuit for the first time. Now one step below the Supreme Court, this case also likely marks the first time that warrantless use of stingrays has reached any federal appellate court.

Stingrays determine a phone’s location by spoofing a cell tower. In some cases, they can intercept calls and text messages. Once deployed, the devices intercept data from a target phone along with information from other phones within the vicinity. At times, police have falsely claimed the use of a confidential informant while in fact deploying this particularly sweeping and intrusive surveillance tool.

The 7th Circuit will now consider a 2013 case known as United States v. Patrick. It involves a Milwaukee man wanted on a probation violation who was suddenly located and arrested by local police with help from the FBI. There is very strong evidence to suggest that he was apprehended through the warrantless use of a stingray.

Patrick’s attorney, Chris Donovan, filed his opening brief in the appeal earlier this month. The case is so notable that the American Civil Liberties Union (ACLU) and the Electronic Frontier Foundation (EFF) also filed an amicus brief earlier this week. The organizations note that the Fourth Amendment of the United States Constitution requires that search warrants demonstrate probable cause of a crime. And, they note, Wisconsin passed a 2014 state law mandating warrants for stingray deployment.

The advocacy groups argue:

Cell-site simulators raise especially serious questions under the Fourth Amendment, and at least require a warrant. Use of a cell-site simulator constitutes a search for several reasons. First, the device can precisely locate and track people’s phones, which requires a warrant for the same reasons that tracking by the service provider does. Second, cell-site simulators transmit probing electronic signals through the walls of homes, offices, and other private spaces occupied by the target and innocent third parties in the area, and thereby force phones to transmit data to the government that reveals where inside those spaces the phones are.

Neil Richards, a law professor at Washington University in St. Louis, told Ars that this type of judicial review is long overdue.

"The Supreme Court has indicated in recent cases that technologies of this sort raise serious constitutional questions. This is why the federal government now requires search warrants before they are used," Richards told Ars in an e-mail. "The fact that [stingrays] are now being used apparently to track probation violations reveals the inevitable ‘mission creep’ in technologies like this, as well as the need for clear constitutional rules to protect our constitutional rights under the Fourth Amendment."

As Richards referenced, last year both the Department of Homeland Security and the Department of Justice (which oversees the FBI) said that they would require a warrant during stingray deployments. However, those policies do not apply to individual state-level law enforcement.