Police in Florida have, at the request of the U.S. Marshals Service, been deliberately deceiving judges and defendants about their use of a controversial surveillance tool to track suspects, according to newly obtained emails.

At the request of the Marshals Service, the officers using so-called stingrays have been routinely telling judges, in applications for warrants, that they obtained knowledge of a suspect's location from a "confidential source" rather than disclosing that the information was gleaned using a stingray.

A series of five emails (.pdf) written in April, 2009, were obtained today by the American Civil Liberties Union showing police officials discussing the deception. The organization has filed Freedom of Information Act requests with police departments throughout Florida seeking information about their use of stingrays.

"Concealing the use of stingrays deprives defendants of their right to challenge unconstitutional surveillance and keeps the public in the dark about invasive monitoring by local police," the ACLU writes in a blog post about the emails. "And local and federal law enforcement should certainly not be colluding to hide basic and accurate information about their practices from the public and the courts."

The U.S. Marshals Service did not respond to a call for comment.

Stingrays, also known as IMSI catchers, simulate a cellphone tower and trick any nearby mobile devices into connecting with them, thereby revealing their location. When mobile phones—and other wireless communication devices—connect to the stingray, the device can see and record their unique ID numbers and traffic data, as well as information that points to the device’s location. By moving the stingray around, authorities can triangulate the device’s location with greater precision than they can using data obtained from a fixed tower location.

The government has long asserted it doesn’t need a probable-cause warrant to use stingrays because the devices don’t collect the content of phone calls and text messages, but instead operate like pen-registers and trap-and-traces, collecting the equivalent of header information. The ACLU and others argue that the devices are more invasive than a trap-and-trace and should require a warrant. By not obtaining a warrant to use stingrays, however, police can conceal from judges and defendant's their use of the devices and prevent the public from learning how the technology is employed.

But the emails released Thursday show police in Florida are going even further to conceal their use of the equipment when they seek probable cause warrants to search facilities where a suspect is located, deceiving the courts about where they obtained the evidence to support their application for the search.

The initial email, which bears the subject line “Trap and Trace Confidentiality," was sent by Sarasota police Sgt. Kenneth Castro to colleagues at the North Port (Florida) Police Department. It was sent after Assistant State Attorney Craig Schaefer contacted police to express concern about an application for a probable cause warrant filed by a North Port police detective. The application "specifically outlined" for the court the investigative means used to locate the suspect. Castro informs his colleague that the application should be revised to conceal the use of the surveillance equipment.

"In the past," Castro writes, "and at the request of the U.S. Marshalls (sic), the investigative means utilized to locate the suspect have not been revealed so that we may continue to utilize this technology without the knowledge of the criminal element. In reports or depositions we simply refer to the assistance as 'received information from a confidential source regarding the location of the suspect.' To date this has not been challenged, since it is not an integral part of the actual crime that occurred."

He then requests that "If this is in fact one of your cases, could you please entertain either having the Detective submit a new PCA and seal the old one, or at minimum instruct the detectives for future cases, regarding the fact that it is unnecessary to provide investigative means to anyone outside of law enforcement, especially in a public document."

Capt. Robert Estrada, at the North Port Police Department, later confirmed in an email, "[W]e have changed the PCA within the agency after consulting with the [State Attorney's Office]. The PCA that was already within the court system according to the SAO will have to remain since it has already been submitted. At some point and time the SAO will submit the changed document as an addendum. We have implemented within our detective bureau to not use this investigative tool on our documents in the future."

The release of the emails showing interference by a state attorney and the U.S. Marshals Service comes two weeks after agents from the Marshals Service took the extraordinary measure of seizing other public documents related to stingrays from the Sarasota Police Department in order to prevent the ACLU from examining them.

The documents, which were responsive to a FOIA request seeking information about Sarasota's use of the devices, had been set aside for ACLU attorneys to examine in person. But hours before they arrived for the appointment to view the documents, someone from the Marshals Service swooped in to seize the documents and cart them to another location.

ACLU staff attorney Nathan Freed Wessler called the move “truly extraordinary and beyond the worst transparency violations” the group has seen regarding documents detailing police use of the technology.

The U.S. Marshals Service is not the only entity conspiring with police to prevent the public from learning about the equipment. The Harris Corporation, a Florida-based company that makes one of the most popular models of stingrays called Stingray, has made law enforcement agencies sign a non-disclosure agreement explicitly prohibiting them from telling anyone, including other government bodies, about their use of the secretive equipment.