The acting head of Immigration and Customs Enforcement (ICE), the federal agency charged with deportations, has confirmed in a new letter that it does not use cell-site simulators, also known as stingrays, to locate undocumented immigrants.

In the August 16 letter, which was sent to Sen. Ron Wyden (D-OR), acting Director Thomas Homan wrote that, since October 2015, ICE has followed similar guidelines put in place by the Department of Justice and the Department of Homeland Security a month earlier, which require a warrant before deploying a stingray.

Homan was responding to an earlier letter than Senator Wyden sent to him. The Oregon Democrat has also recently sent a similar letter to the Department of Justice, which has not yet responded. That August 1 letter states: "We are concerned that the Department may not be adequately disclosing to courts important details about how stingrays work and their impact on innocent Americans."

As Ars has reported for years, stingrays determine a phone’s location by spoofing a cell tower. In some cases, they can also intercept calls and text messages. Once deployed, the devices intercept data from a target phone along with information from other phones within the vicinity.

Historically, government agencies and the companies that make the devices have been extremely reticent about providing precise details about how the devices work. The Harris Corporation, which makes the trademarked device known as the StingRay, among other related surveillance devices, has previously refused to provide additional details to Ars.

The Homan letter makes a point though to say that “interference with non-targeted mobile devices is virtually nonexistent.”

As he continued:

The mobile identifier of non-targeted mobile handsets is verified by the cell-site simulators automatically as a non-target and sent back to the best mobile network. This process is conducted in an amount of time that is not noticeable to the user. If a call is placed or received during the exact instant the verification is taking place, interference may result. The equipment software has provisions to allow a captured device one currently engaged by the cell-site simulator to return to the host network if the captured device initiates a call. In all circumstances, devices are always able to dial 911 without any disruption of service.

This description is curious given that a Friday warrant application, filed by an FBI agent in a drug case in Wisconsin, describes “service disruption” to phones that aren’t targeted by the stingray as being “ brief and temporary .”

Homan’s assertion about “always” being able to call 911 regardless of whether a stingray is in use also appears not to be consistent with an episode in Canada from May 2016. At that time, Canadian media reported on the Asian Assassinz trial in Toronto. In that case, Detective Shingo Tanabe swore in an affidavit that the Toronto Police would not keep its stingray on for more than three minutes at a time for fear of running afoul of Canadian telecom law and blocking possible 911 traffic.

In August 2016, a Georgetown law professor filed a formal complaint to the Federal Communications Commission over the alleged disruption to the 911 system.

“I appreciate this substantive response from ICE, confirming the agency’s policy is to obtain a warrant to use stingrays and other cell-site simulators,” Sen. Wyden, who has been pushing since 2011 for a law to require warrants when stingrays are used, wrote to Ars in a statement. “But department policies aren’t the same as lasting protections for our liberties.”