What is used by dozens of local law enforcement agencies around the country, featured in numerous news stories , and discussed in court , yet treated by the FBI like it is top secret? That would be "Stingray" cell phone surveillance gear, of course.

This week, MuckRock released a mostly redacted copy of the nondisclosure agreement that the FBI makes local police departments sign before they are permitted to buy a Stingray from the Florida-based Harris Corporation. We have seen the FBI pressure local law enforcement agencies to withhold basic information about purchase and use of Stingrays before, but now we have greater insight into how it does so.

According to the agreement , released by the Tacoma, Washington, Police Department, the Harris Corporation notifies the FBI whenever a local police department wants to buy a Stingray, and then the FBI requires the local agency to sign a lengthy nondisclosure agreement before buying the device. The FBI's role in the process is a condition of the Federal Communication Commission's equipment authorization issued to the Harris Corporation.

The result is that members of the public, judges, and defense attorneys are denied basic information about local cops' use of invasive surveillance gear that can sweep up sensitive location data about hundreds of peoples' cell phones. For example, when we sought information about Stingrays from the Brevard County, Florida, Sheriff's Office, they cited a non-disclosure agreement with a "federal agency" as a basis for withholding all records. When the ACLU of Arizona sued the Tucson Police Department for Stingray records, an FBI agent submitted a declaration invoking the FBI nondisclosure agreement as a reason to keep information secret.

In fact, the FBI agent's declaration in the Arizona case quotes extensively from the nondisclosure agreement, thus filling in some of the text blacked out in the Tacoma document. The Tacoma Police Department can't redact text from an FBI document that the FBI has already disclosed in court proceedings.

According to the Arizona declaration, the nondisclosure agreement states that:

Disclosing the existence of and the capabilities provided by [cell site simulator equipment] 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 employ countermeasures to avoid detection by law enforcement.

Therefore, "any information" about Stingrays "shall be protected from potential compromise by precluding disclosure…to the public."

Extensive information about Stingray devices and other forms of cell phone tracking is already in the public domain. Withholding basic data about how Stingrays are used, what rules govern their use, how the privacy of innocent bystanders is protected, and whether police are getting probable cause warrants from judges doesn't keep us any safer. It only functions to subvert our right to keep tabs on what our government is up to. This kind of secrecy allows the government to keep constitutional violations hidden in the shadows.

In the midst of the government's excessive secrecy, the ACLU continues to push for transparency and reform. On Monday, the ACLU of Florida filed a motion to unseal applications submitted by the Sarasota Police Department seeking court orders authorizing Stingray use, as well as the resulting orders. Those records had originally been taken from the court by the police and kept in the police station—an unusual and illegal practice—and then, incredibly, spirited away by the U.S. Marshals Service when the ACLU filed a public records request. A judge subsequently instructed the government to deposit sealed copies of the applications and orders back with the court. We are asking for them to be made public.

And in Texas, the ACLU's Chris Soghoian testified at the state legislature about Stingrays and other police surveillance tactics, prompting one local paper to dig into the Houston Police Department's purchase of hundreds of thousands of dollars' worth of Stingray equipment.