The Boston Police Department is keeping the public largely in the dark about how it uses covert cellphone trackers — devices that have raised civil liberties questions across the country.
The department declined to provide details about how the trackers are used in the field, saying that disclosing protocols or the types of cases that they are being used for would endanger lives. The trackers allow police to identify all phones at a given locale and to log the movements of handsets over time.
Other law enforcement agencies have provided details of tracker usage, including the New York Police Department, which released documents to the New York Civil Liberties Union that were made public this month. The documents showed that the NYPD had used the trackers at least 1,000 times since 2008, for investigations ranging from homicide and rape to identity theft and missing-person searches.
Documents obtained by the New England Center for Investigative Reporting under the state Public Records Law show the Police Department and the Suffolk district attorney's office have agreements with the Federal Bureau of Investigation not to disclose information about the trackers. The devices are also known as StingRays, the brand name of the most widely used models.
Under the pacts, police and prosecutors agreed not to provide information about the devices, even in court proceedings, without prior approval of the FBI. They also agreed to seek dismissal of a case, at the FBI's request, rather than disclose sensitive information about them in court.
The US public defender's office in Boston, which represents indigent defendants in federal prosecutions, says that prior to a New England Center for Investigative Reporting story in November, it was unaware that any such agreement existed. The center revealed the Boston Police Department's use of the devices.
"The danger is that nondisclosure agreements foster mischief," said William Fick, an assistant US public defender. He said such agreements "create perverse incentives" for law enforcement and prosecutors to omit or misrepresent how they obtained information.
The Boston Police Department says it typically obtains search warrants to monitor cellphones. But detectives are not required to spell out in a warrant application that they intend to use a tracker in their search, a police spokesperson said.
Civil liberties advocates have argued that without knowledge that a tracker will be used, judges are less likely to put limits on warrants to keep police from gathering cellphone information about someone who is not the target of the investigation.
The trackers are called cell-site simulators because they monitor mobile devices by mimicking cell towers. When the tracker is turned on, all cellular phones within range connect to it and transmit their location.
"Unless the use of a cell-site simulator is disclosed, a defendant cannot raise a legal or constitutional challenge seeking to suppress evidence on that basis," Fick said.
Prosecutors "weighed both the public safety potential and the practical realities of the technology" before agreeing to the nondisclosure terms, said Jake Wark, a spokesman for the Suffolk district attorney. "There is an obvious public safety benefit to be able to quickly and accurately locate a person who is in danger or poses a threat."
The agreement requires prosecutors and police to notify the FBI if any court proceeding might disclose sensitive information about the capabilities of cellphone trackers. Both parties agreed to seek dismissal of such cases at the FBI's request.
Neither the police nor the district attorney has sought to dismiss a case on these grounds, Wark said.
The trackers don't come cheap. Documents released to the New England Center for Investigative Reporting in early February include an April 2014 cost estimate for purchasing a variety of components from Harris Corp., the Florida-based manufacturer of StingRays.
The $333,910 price quote matches the sum of a series of entries in the city's online checkbook for police expenditures on July 8, 2014.
The Boston Police Department confirmed that it acquired the equipment in July 2014.
But the department has not answered inquiries about the particular device and possible components it purchased. The model number provided by the Police Department corresponds to the KingFish, a hand-held version of the StingRay tracker, made by Harris Corp.
Police purchased the equipment using a portion of an $8.6 million grant from the Department of Homeland Security, according to Laura Oggeri, a spokeswoman for the Boston mayor's office.
Edward Callahan, head of the Police Department's technology bureau, told Boston City Councilor Ayanna Pressley in May that use of the trackers was subject to obtaining a warrant. They may be used without warrants when "exigent circumstances exist that require immediate law enforcement action," Callahan wrote in a letter to Pressley.
In 2015, Washington enacted a law that requires explicit search warrants for tracker use and includes provisions for police to minimize data collection from bystanders' cellphones. Similar laws have been enacted in Virginia, Minnesota, Wisconsin, and Utah.
The Justice Department amended its protocol in September to require a search warrant to track phones via cell-site simulators, instead of vague court orders. The Department of Homeland Security followed suit in October. Both policies now require federal agents to be explicit about what technology will be used in the search.
Judges may not have the same clarity when reviewing Boston police warrant requests. Lieutenant Michael McCarthy, a spokesman for the Police Department, said "there is no requirement" that affidavits in support of such requests "specifically indicate" that the trackers will be used.
He said he couldn't provide examples of cellphone tracking warrants.
Police must be explicit about how and when "they intend to conduct invasive electronic monitoring" to make sure people who aren't targets "aren't swept up in police spying operations," said Kade Crockford, who researches surveillance issues for the American Civil Liberties Union of Massachusetts.
The Suffolk district attorney's office has described cellphone trackers as a "tool that can protect and save lives by apprehending a violent fugitive or rescuing a kidnap victim."
But the local ACLU chapter says it's worried that police may be using trackers on routine criminal investigations.
Police officers in Baltimore used trackers in probes of murders and armed robberies, but also in low-level theft and harassment cases, according to a surveillance log released last year.
McCarthy said the Police Department has no list of tracker cases to provide, and declined to give any examples of their use.
"The types of cases can vary and the decisions to apply for its use are based on the circumstances of each individual case," McCarthy said.
The New England Center for Investigative Reporting (www.necir.org) is a nonprofit news outlet based at Boston University and WGBH (PBS/NPR) in Boston. New England Center for Investigative Reporting intern Amanda Lucidi also contributed to this report.