On Wednesday, the Maryland Court of Special Appeals published a legal opinion finding that state police must not only obtain a warrant before deploying a cell-site simulator, but are required to also fully explain to the court what exactly the device does and how it is used.

As Ars has long reported, cell-site simulators—known colloquially as stingrays, can be used to determine a mobile phone’s location by spoofing a cell tower. In some cases, stingrays 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 when they have actually deployed these particularly sweeping and intrusive surveillance tools.

In recent years, stingray use has come under increasing scrutiny, with several states including California, Washington, Virginia, Minnesota, and Utah now mandating a warrant be issued for their use. Last year, the Department of Homeland Security and the Department of Justice also imposed new policies that require a warrant for stingray use in most cases.

In an e-mail to Ars, American Civil Liberties Union attorney Nathan Wessler called Wednesday's opinion the "first appellate opinion in the country to fully address the question of whether police must disclose their intent to use a cell site simulator to a judge and obtain a probable cause warrant."

"The court’s opinion is a resounding defense of Fourth Amendment rights in the digital age," he continued. "The court’s withering rebuke of secret and warrantless use of invasive cell phone tracking technology shows why it is so important for these kinds of privacy invasions to be subjected to judicial review. Other courts will be able to look to this opinion as they address rampant use of cell site simulators by police departments across the country."

The case, known as State of Maryland v. Andrews, involves a Baltimore man, Kerron Andrews, who was accused of attempted murder in connection to a shooting in April 2014. As part of his legal defense, his lawyers pressed the government to disclose exactly how he was located inside a home and arrested on May 5, 2014. It later came out that Baltimore police detectives used a "Hailstorm," a specific model of cell-site simulator.

In August 2015, Maryland Circuit Court Judge Kendra Ausby ruled in favor of the defense’s request to suppress all evidence obtained as a result of the use of the stingray. Prosecutors then appealed the case up to the Court of Special Appeals, which has now upheld that ruling. Absent the evidence obtained via the stingray, further prosecution of this case will be nearly impossible, unless the Maryland Court of Appeals takes up the case.

As the Maryland Court of Special Appeals concluded:

We determine that cell phone users have an objectively reasonable expectation that their cell phones will not be used as real-time tracking devices through the direct and active interference of law enforcement. We hold, therefore, that the use of a cell site simulator, such as Hailstorm, by the government, requires a search warrant based on probable cause and describing with particularity the object and manner of the search, unless an established exception to the warrant requirement applies.

“We are troubled”

The court also concluded that the government could not rely on a pen register application, which it had obtained. Authorities often opt for pen registers because, under federal law, these are granted under a very low standard. Authorities must simply show that information obtained from the pen register is "relevant to an ongoing criminal investigation"; they don't need the probable cause required for a warrant. In this case, police appear to have taken the permission to use a pen register as permission to also deploy a stingray, which has far more capability than the simple "trap and trace" functionality offered by a pen register.

As the three-judge panel found:

Here, the State inserted language into its application and proposed order attempting to, without being specific, obtain court authorization for more than a pen register trap & trace order. Although the application does request authorization to use a "Cellular Tracking Device," it fails to name or describe any cell site simulator. In fact, there is absolutely nothing in the application or order that identifies the Hailstorm device, or provides even a rudimentary description of cell site simulator technology. The application also failed to identify any geographical limitation to the BPD’s use of the undisclosed surveillance technology, and did not explain what was to be done with the information collected. Nor did the application disclose the possibility that the technology employed may capture the cell phone information (unique serial numbers) of innocent third parties in range of the target area. Finally, we are troubled that the application for a pen register trap & trace order did not fully apprise the circuit court judge from whom it was sought of the information that it would yield. Based on the application that he received, the circuit judge was entitled to expect that the results would be a list of telephone numbers that Andrews called and that called Andrews—not a real-time fix on his location. We determine that the pen register trap & trace order in this case failed to meet the requirements of a warrant.

The 74-page opinion also excoriated what the Baltimore Police Department, like many other law enforcement agencies nationwide have done: concealed the use stingrays via a nondisclosure agreement signed between them and manufacturer Harris Corporation.

The analytical framework requires analysis of the functionality of the surveillance device and the range of information potentially revealed by its use. A nondisclosure agreement that prevents law enforcement from providing details sufficient to assure the court that a novel method of conducting a search is a reasonable intrusion made in a proper manner and "justified by the circumstances," obstructs the court’s ability to make the necessary constitutional appraisal.

The court added that this nondisclosure agreement remains "inimical to the constitutional principles we revere."

A related case examining the warrantless use of stingrays, known as United States v. Patrick, is currently pending before the 7th Circuit Court of Appeals in Chicago. That ruling, which could come down at any time this year, would be the first such ruling in a federal appeals court.

The Maryland State’s Attorney’s Office did not immediately respond to Ars’ question as to whether it would be appealing this ruling further, to the Maryland Court of Appeals, the state's highest court..