The head of the Internal Revenue Service told a Senate committee on Tuesday that its stingrays are "only used in criminal investigations."

The remarks came just one day after The Guardian revealed that the IRS had purchased the devices in 2009 and 2012.

Stingrays, also known as cell-site simulators, can be used to determine a phone’s location by spoofing a cell tower, and in some cases they can intercept calls and text messages. Once deployed, the devices intercept data from a target phone as well as information from other phones within the vicinity.

Many federal and local law enforcement agencies, including the FBI and the Department of Homeland Security, use the devices—but it's somewhat surprising that America’s top tax cop is also a user. Few agencies have been particularly forthright in describing exactly what the capabilities of the devices are and how they are used and under what circumstances.

"It is only used in criminal investigations," Commissioner John Koskinen testified before Sen. Ron Wyden (D-Ore.) and the rest of the Senate Finance Committee. "It can only be used with a court order. It can only be used based on probable cause of criminal activity. 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. You may pick up texting. But what I would stress is that it does follow Justice Department rules. It requires a court order. It requires probable cause with regard to criminal investigations. It is not used in civil matters at all. It is not used by other employees of the IRS."

Koskinen was unable to provide the number of times that stingrays had been used, but he promised that he would do so in writing to Sen. Wyden within 30 days.

As far as the IRS' current practice goes, it's worth noting that a court order, typically written under the guise of the federal pen register statute, carries a lower legal burden than a warrant, which requires a showing of probable cause. IRS spokesman Bruce Friedland did not respond to Ars' question as to why—if it has probable cause of criminal activity—the IRS does not always file warrant applications when it wants to deploy stingrays.

However, a former top IRS official who has been out of the agency for several years told Ars, "I only know that to my knowledge and other very experienced tax counsel in the US, we have never seen it in a tax case."

This technique of opting for the pen register is not new. In the pre-cellphone era, a "pen register/trap and trace order" allowed law enforcement to obtain someone's calling metadata in near real-time from the telephone company. These days, that same data can also be gathered directly by the cops themselves through the use of a stingray. In some cases, police have gone to judges asking for such a device or have falsely claimed a confidential informant while in fact deploying this particularly sweeping and invasive surveillance tool.

Most judges are likely to sign off on a pen register application, not fully understanding that police are actually asking for permission to use a stingray. Under federal law, pen registers are granted under a very low standard: authorities must simply show that the information obtained from the pen register is "relevant to an ongoing criminal investigation."

Again, getting a judge to sign off on a pen register is a far lower standard than being forced to show probable cause for a search warrant or wiretap order. A wiretap requires law enforcement to not only specifically describe the alleged crimes but also to demonstrate that all other means of investigation have been exhausted or would fail if they were attempted.

The IRS commissioner's remarks came less than a week after the Department of Homeland Security, which oversees various agencies including the Secret Service and Customs and Border Protection, said that it would implement a new formal policy on stingray use.