Boston Police have confirmed that they deploy devices capable of secretly locating cell phones in real time, a controversial practice that critics say impinges on the public's privacy and constitutional rights.

Former Boston police commissioner Edward Davis signed an agreement with the FBI in 2013 to obtain the cell phone tracking systems, documents released to a privacy activist yesterday in response to a public records request reveal.

This is the first official confirmation that BPD is one of dozens of law enforcement agencies nationwide that deploy the controversial technology.

While law enforcement officials say cell phone trackers are critical to fighting crime and thwarting terrorism, critics say that a lack of transparency has allowed surveillance to drift into routine investigations without open debate. In other cities, such as Baltimore and Chicago, judges and public defenders have raised concerns over being kept in the dark about cell phone tracking.

"I think that our elected officials, who represent us, cannot make informed decisions about technology that has substantial privacy implications if they know nothing about it," says Mike Katz-Lacabe, the privacy activist who received the documents from BPD by mail yesterday.

The BPD has not responded to questions regarding how often it deploys cell phone trackers or for which types of investigations, nor have they provided documentation for how many cell phone trackers the department owns.

Cell phone tracking systems are used widely to locate sources of 911 calls. But police can use more targeted trackers to identify suspects or monitor an individual’s movements with precision.

Boston police officials insist that disclosing any information about its cell phone trackers will undercut public safety. But after two protracted public records fights — one by this reporter and the other by Katz-Lacabe, who is based in California — the department relented and released a document concerning their deployment.

By signing a six-page agreement, Boston police received federal approval in June 2013 to purchase devices, known as cell site simulators, that imitate cell phone towers. The simulators force nearby mobile devices to connect, giving police the specific location of each cell phone. The systems are also known as “StingRays”, after the most popular brand.

The FBI has used cell site simulators since at least 1995, and closely guards the technology’s particulars. As part of an arrangement between the FBI and Harris Corporation, the Florida-based manufacturer of the StingRay product line, any state or local law enforcement agency that seeks to buy a system must sign a nondisclosure agreement with the FBI.

Boston's nondisclosure agreement is now in the public domain as a result of the two records requests. Among other provisions, the nondisclosure agreement bars the department from sharing any information regarding cell phone trackers and their deployment with the public or any non-law enforcement entity.

“Disclosing the existence of and the capabilities provided by such equipment/technology to the public would reveal sensitive technological capabilities possessed by the law enforcement community and may allow individuals who are the subject of investigation [...] to avoid detection by law enforcement,” the agreement reads.

Perhaps the most controversial provision requires the BPD to notify the FBI if any prosecutor intends to disclose sensitive information in court, and to seek dismissal of those cases if the FBI determines that such disclosure might compromise the technology.

Former commissioner Davis signed the nondisclosure agreement in late June 2013, the document indicates, a few weeks after the Boston Marathon bombing. A handful of BPD detectives also signed the nondisclosure, along with a state police lieutenant.

Over the past year, reporters and civil liberties advocates obtained identical documents from law enforcement in Chicago, Tacoma, Minnesota and more than a dozen other jurisdictions. The New York Civil Liberties Union obtained the first unredacted copy in April, following a lawsuit against the Erie County Sheriff’s Office under the New York public records statute.

The FBI refuses to indicate which agencies nationwide have signed such an agreement, or to indicate how many have done so. In June 2014, the Massachusetts State Police told the ACLU of Massachusetts that it does not own any cell site simulators.

Boston police vigorously attempted to rebuff numerous requests for records on its cell phone trackers. Notably, in letters rejecting such requests, the BPD quoted heavily from the nondisclosure agreement itself.

In February, BPD lawyers broadly claimed that all records regarding cell site simulators were immune from the Massachusetts public records law because their release "would prejudice the possibility of effective law enforcement." When the state records authority demanded additional justification, BPD responded that releasing documents would render cell phone trackers “essentially useless.”

The state found the department’s argument insufficient, and ordered the documents released in late September. In a letter dated November 6, the BPD grudgingly released the nondisclosure agreement to Mike Katz-Lacabe, an activist based in San Leandro, California.

“Contrary to the Department's position, the Public Records Division of the Commonwealth has allowed your appeal and ordered production of the requested materials,” wrote the BPD’s Nicole Taub.

Katz-Lacabe posted the document online yesterday. Civil liberties groups in Massachusetts hailed the disclosure as an initial step.

“We're thrilled that the BPD has finally released this document to the public, where it belongs," says Kade Crockford of the ACLU of Massachusetts. “We look forward to learning more about how BPD has used StingRays, including in which kinds of investigations, for what purposes, and with what legal authority.”

Last year, Black Lives Matter activists in Chicago alleged that police used a StingRay to monitor their protest. In Baltimore, police have used their cell site simulators on investigations of a broad range of crimes, from kidnapping to petty theft, according to an investigation by USA Today. Baltimore public defenders are reviewing more than 2,000 cases where police may not have disclosed use of the devices to suspects’ lawyers.

Both the Justice Department and the Department of Homeland Security have revised guidelines in recent months to address how their agents can track cell phones. Except under extraordinary circumstances, the new rules require obtaining a warrant, which was not previously the standard. California, Virginia and Washington enacted legislation this year to require warrants for cell phone tracking by police, as did Minnesota and Utah in 2014.

BPD has yet to provide details regarding the department's protocols for deploying cell phone trackers as part of investigations.