When federal prosecutors told a Baltimore defense attorney this month that authorities had used a high-tech device to track his client, it was more than a simple step in the legal discovery process.
It was a rare admission by law enforcement that they had used a controversial device known as a stingray, a cellphone-tracking tool that authorities have gone to great lengths to conceal.
Developed for the military, the stringray mimics a cell tower to pin down the location of a phone to within a few feet. Because it is mobile, officers can drive around until they get a signal from the target phone.
The device — known generically as an IMSI catcher or a cell site stimulator — has proved effective in finding suspects. But advocates for privacy and civil liberties say the secrecy surrounding its use makes it impossible to determine whether officers have secured the necessary approval, or have stayed within its bounds.
They are also concerned about the rights of other cellphone users whose data might be caught up in a search. The Electronic Frontier Foundation has called the stingray "the biggest technological threat to cellphone privacy you don't know about."
As a rule, federal authorities do not talk about the device. Police departments across the country have said they are prevented by nondisclosure agreements from revealing when they have used one.
The Baltimore Police Department has owned one of the devices since at least 2009, according to budget records. But it is only recently that defense attorneys have begun to demand details.
Last week, a Baltimore circuit judge threatened to hold a city detective in contempt for refusing to say how police track phones. Prosecutors sidestepped the threat by agreeing to withdraw the evidence police had gathered.
It was the second time since September that prosecutors had given up evidence rather than divulge the details of how it was obtained.
At least four police departments in Maryland — Baltimore's, and Anne Arundel, Montgomery and Prince George's counties' — have acquired a stingray. None of them will say much more.
"We have a cell site simulator that is used as an investigative tool to assist in locating critical missing persons and wanted felons," Anne Arundel police Lt. T.J. Smith wrote in response to questions from The Baltimore Sun. "To reveal further details of the investigative techniques would jeopardize ongoing and future investigations. We utilize the technology in a responsible and lawful manner."
A review of court cases involving the Baltimore Police Department's Advanced Technical Team, which is deployed to track and listen to electronic communications, shows that the unit is regularly able to pinpoint a phone down to a specific apartment or moving vehicle. But the team often does not reveal how.
"It speaks to the tremendous and corrosive secrecy surrounding police use of these that we haven't seen more challenges in court," said Nathan Freed Wessler of the national ACLU office.
In one case, a Baltimore detective testified that "Homeland Security" prevented him from discussing the department's methods.
Last spring, when a state lawmaker at a hearing in Annapolis asked a Maryland state trooper whether his agency used a stingray, the trooper said the answer is "classified."
That didn't sit well with Sen. Christopher B. Shank, a Washington County Republican, who said he wants to know more about the technology.
"If they were using this in a partnership to deal with homeland security threats, then I'd be more willing to accept that answer," Shank said in an interview. "But when we're hearing that for run-of-the-mill crimes, that doesn't quite pass muster with me."
Attorney John Deros said it was "weird" how Baltimore police pinned a cellphone robbery on his client last year.
They said they zeroed in on the suspect as he stood on a sidewalk. Officers approached and said they were doing a check for "wanted" individuals. Then they called the phone so it would ring.
Court records said only that police had "developed" his client as a suspect and put him into a photo lineup to show to the victim. Deros said he was shown no court order to indicate how police pinpointed the phone's location.
Someone who takes a phone during a robbery cannot expect police not to try to track it, Deros said. But he said police should disclose the technology they used.
"If you're not doing anything wrong, then tell us what you're doing."
Privacy advocates are filing court challenges and public records requests seeking more information.
The ACLU was able to obtain emails in which local police in Florida said federal marshals told them to obfuscate the use of the stingray when explaining investigations and to refer instead to "information from a confidential source." In Tacoma, Wash., a reporter obtained a heavily redacted nondisclosure agreement with the FBI that agencies must "coordinate" with the agency and not release details publicly.
And in North Carolina last week, a judge unsealed hundreds of documents that showed police in the state sought permission to use the stingray an average of twice a week since 2010. The records had been shielded even from prosecutors, who are reviewing them.
Prosecutors have argued that the technology is too effective to detail in public and that language in court orders clearly lays out what investigators are doing. Local authorities say there's no evidence of the technology being abused.
A stingray is not the only way to track a cellphone. Police can seek a court order to require a phone company to "ping" a phone and triangulate its location. Investigators can also get historical data to show the towers a cellphone connected with, a method used regularly to place a suspect at the scene of a crime.
This year, the General Assembly raised the bar for police in Maryland who want to track phones in real time, with a law that requires authorities to show probable cause and obtain a search warrant.
Police and prosecutors testified against the bill, saying that phone tracking is often used to find missing persons, stolen property and potential witnesses. Baltimore police said they use real-time tracking so often that they would need to spend $100,000 to hire two staff members to comply with the requirements.
Prosecutor Wesley Adams testified that investigators often track phones to develop leads before they are ready to present probable cause for a warrant.
"You're eliminating our ability to investigate people," Adams, who was elected this month as Anne Arundel County state's attorney, said in an interview.
Because police refuse to talk about stingrays, it is unclear whether they are obtaining warrants under the new law to use the devices. Shank said they should, adding, "That was clearly our intent."
No federal appeals court has reviewed the legality of the stingray. But the government scored a major victory in 2013, when a federal judge allowed the use of evidence the FBI had collected on an alleged Arizona hacker who was located with a stingray.
Peter Nothstein, a former federal prosecutor, said that in his experience, the devices were used only occasionally, and always with court approval. He said the evolving debate over privacy likely means that the secrecy around the technology "will not be able to continue."
The case in which prosecutors acknowledged that law enforcement had used a stingray involved a murder-for-hire sting hatched by detectives. Agents proposed a fictitious murder to an alleged hit man and gave him a phone. They used the stringray to track the phone to the home of another man. Both are now charged in an alleged murder-for-hire conspiracy.
For that case, authorities obtained a pen register order. Federal authorities have said the stingray falls under the parameters of a pen register, a device that records a dialed number, and does not require a warrant.
C. Justin Brown, an attorney for one of the men charged in the alleged conspiracy, argued in a legal filing that the court order misleads judges who are not always familiar with the technology. Use of the stingray "was a search of [client Robert] Harrison's apartment, of the phone, and of Harrison's person," Brown argues, which he says required a warrant.
Brown said he knows of the stingray only because its use was disclosed by an FBI agent. Federal prosecutors later filed an acknowledgment that included a description of how the technology works.
Prosecutors aren't saying more. Assistant U.S. Attorney James Warwick wrote in a court filing that the stingray is "subject to the privilege for sensitive law enforcement techniques."
Wessler said that argument rings hollow. "This is an investigative technique just like dozens of other things police do. The fact that criminals know police have fingerprint kits or infrared cameras or handcuffs, that doesn't destroy law enforcement's ability to do its job."
Defense attorney Joshua Insley has been pressing Baltimore police about whether they used a stingray to find his client in a robbery case. Detective John Haley testified last week that no stingray was used, but said he was barred by a nondisclosure agreement from disclosing his unit's phone-tracking capabilities.
Circuit Judge Barry G. Williams said he would hold Haley in contempt if he did not provide more information. Prosecutors agreed to withdraw the evidence.
Insley believes authorities are hiding their use of the device. He plans to press forward.