Law enforcement agencies on Thursday met with Illinois lawmakers about legislation that would limit the use of new technology that tracks cell phone use.

The device, called a Stingray, has purportedly been used for surveillance by the Chicago Police Department during protests and rallies, but the department has been secretive about exactly how extensively its used. It has raised questions about invasion of privacy that civil libertarians and lawmakers are trying to answer.

“Stingrays” are small devices that mimic cellphone towers. They are typically carried in police vehicles that have antenna that attract every cellphone signal in the area.

“The towers are able to capture the serial information and identify every phone that is in the area,” said Ed Yohnka of the ACLU of Illinois. “So even if the police are looking for a certain phone or tracking an individual, in the course of an investigation, they sweep up all of the identifying information about everyone who is in that particular area.”

Yohnka says that law enforcement agencies primarily use Stingrays as tracking devices, but he’s concerned that they are capable of doing much more.

“It’s able to capture the serial number, it’s now able to capture the content of what’s on the phone, who the phone has called, what numbers have been dialed, what numbers are stored, what websites have been visited, text messages sent and received, and it can actually send things to the smart phone. So one of the new tools of this technology is to send a signal that drains the battery, or blocks the phone, or places malware on the phone so that the government or a law enforcement agency can control the phone,” Yohnka said.