The ACLU issued a release saying the device, known as a "stingray," fools wireless devices into seeing it as a cell tower, allowing the stingray to spy on communications, KNTV, San Francisco, reported Friday.
The release said the devices are used in California by the San Francisco Police Department, the San Jose Police Department, the Fremont Police Department, the Oakland Police Department and the Alameda County District Attorney's Office.
"Stingrays create serious privacy concerns because they collect information about the devices and whereabouts of innocent third parties, and not just the target of an investigation," the ACLU said.
The Oakland Police Department released a statement confirming officers have used the stingray device "to assist us in a variety of investigative strategies."
An investigative report from the Oakland police revealed the department made 21 arrests in 2007 stemming from use of the stingray or electronic surveillance.
The ACLU said it is seeking more information about how the stingray is used in investigations and whether the process involves obtaining warrants.