ROBERT SIEGEL, HOST:

Now a question about the way police do their jobs. When is it OK for them to keep secrets about the tools they use - tools that can spy on us? That's being asked more and more in connection to something called IMSI catcher. And IMSI catcher is a device that police use to intercept cell phone signals, and as NPR's Martin Kaste reports, the police would rather keep the devices out of the public eye.

MARTIN KASTE, BYLINE: IMSI stands for international mobile subscriber identity. Basically, it's your phone's ID. IMSI catchers are portable devices that track phones by pretending to be cell towers.

NATE FREED WESSLER: They send out a signal that's more powerful than the actual cell phone towers in the area and trick every phone in the area into reporting back its identifying information, including its electronic serial number and also its location.

KASTE: That's Nate Freed Wessler, an attorney with the ACLU. He's been on the trail of IMSI catchers for a while, trying to piece together a more complete picture of how the police use the devices. That's easier said than done.

WESSLER: We've seen local police departments across the country trying very hard to keep even extremely basic information about their use of this technology secret.

KASTE: For instance, earlier this summer the ACLU requested records about IMSI catcher use at a Police Department in Florida. Wessler says the ACLU had the state's public records law on its side, but then the U.S. marshals intervened.

WESSLER: While we were negotiating with the police about whether we could see them, the marshals actually swooped in from their nearest field office, grabbed the boxes of records from the Sarasota police, and spirited them away to an undisclosed location.

KASTE: The ACLU did get internal E-mails in which the police discussed how they could obscure their use of IMSI catchers when they wrote court documents. Federal agencies just won't discuss the technology, and the secrecy extends even to the company that makes the best-known IMSI catcher, a model called The Stingray. The Harris Corporation says it can't comment on technology that the company, quote, "may or may not make."

All of this might lead you to believe that IMSI catchers are super-advanced pieces of spy gear, but apparently they're not.

(SOUNDBITE OF ARCHIVED RECORDING)

CHRIS PAGET: Welcome to practical cell phone spying.

KASTE: This is a video of a hacker named Chris Paget at a hacker conference. He's demonstrating his homemade IMSI catcher.

PAGET: If you do not want your cell phone calls recorded, turn your phone off.

KASTE: And if you're not much of a do-it-yourself-er, you can always buy a ready-made professional grade IMSI catcher online or overseas. Crooks and spies are now using them so much, the FCC has put together a task force to try to get a handle on the problem, all of which makes it that much stranger when the police try to obscure the fact that they have the same devices.

STEPHANIE PELL: This isn't a secret.

KASTE: Stephanie Pell has been researching this at the Army Cyber Institute at West Point, though her views here are not necessarily those of the Army. A former prosecutor, Pell says the devices are a legitimate tool for the police, but she's dismayed by the continued secrecy. It's no longer hiding what the devices can do, she says. It's just hiding how they're used.

PELL: The only person or people who know what was collected is the IMSI catcher operator. It makes after-the-fact oversight very difficult.

KASTE: Are police keeping data from non-targeted phones? Are they getting warrants? Should they? She says it's going to be hard for the courts and lawmakers to set specific rules for IMSI catchers as long as the police try to pretend that they don't use them. Martin Kaste, NPR News. Transcript provided by NPR, Copyright NPR.