Two top senators are probing use by the Internal Revenue Service of secret cellphone tracking systems that are more often utilized by federal or local law enforcement agencies.

IRS Commissioner John Koskinen admitted this week that the agency does use the technology, known as cell-site simulators, or StingRays. The admission came after a report by The Guardian that indicated the IRS has spent more than $71,000 to upgrade a version of the device and to receive training from a company that manufactures the devices.

Senate Judiciary Committee Chairman Chuck Grassley and ranking member Patrick Leahy on Thursday sent a letter to Treasury Secretary Jacob Lew demanding answers about the use of the technology by the IRS.

“We were surprised to learn that IRS investigators may be using these devices,” Mr. Grassley, Iowa Republican, and Mr. Leahy, Vermont Democrat, wrote in the letter. “While the devices can be useful tools for identifying the location of a suspect’s cell phone or identifying an unknown cell phone, we have previously expressed concerns about the privacy implications of these devices.”

Cell site-simulators work by mimicking cellphone towers to trick cellphones to connect to them, enabling investigators to obtain identifying information about the phones and their locations. Law enforcement officers often deploy the suitcase-sized StingRays by hauling them around in vehicles as they drive through neighborhoods looking for a suspect’s phone, scooping up data on the cellphones of any passers-by in the process.

Mr. Koskinen testified Tuesday before the Senate Finance Committee that the devices are only used in criminal investigations and not for any civil matters.

“What it does is to primarily allow you to see point-to-point, where communications are taking place. It does not allow you to overhear — the technique doesn’t — voice communications,” Mr. Koskinen said. “You may pick up texting. But what I would stress is that it does follow Justice Department rules. It requires a court order.”

Law enforcement agencies have in the past attempted to deploy the technology in secret, with police departments willing to go as far as to drop criminal charges rather than disclose investigative methods. But criticism over their secretive use has prompted outcry over potential privacy violations, and federal law enforcement agencies have in recent months developed new protocol for their use.

The Justice Department in September announced new regulations that would require investigators to obtain warrants supported by probable cause before using the devices. The Department of Homeland Security followed suit releasing similar regulations this month.

Among the outstanding information the senators are looking to gather about the IRS’s use of the technology is how many devices the agency has, how often it has deployed the devices, and what type of internal protocols are in place to restrict its use.

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