Lawmakers on the House Oversight Committee sent letters Monday to the heads of 24 federal agencies seeking answers about the use of a controversial surveillance technology.

The devices, known by the brand name “StingRay,” simulate a cell phone tower and are able to collect information on mobile phones and their users. Lawmakers say they are trying to create a comprehensive record of how different federal agencies use the devices. The scrutiny comes after the Department of Justice and the Department of Homeland Security adopted new policies for how their employees may deploy the technology.

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“As it was with DOJ and DHS before those agencies issued department-wide policies governing use of the devices, the Committee is concerned that other federal agencies may be governed by a patchwork of policies,” the letters said. “Those policies may permit the use of cell-site simulators devices through a lower standard than a search warrant obtained after a showing probable cause."

The letters, signed by Oversight Chairman Jason Chaffetz Jason ChaffetzThe myth of the conservative bestseller Elijah Cummings, Democratic chairman and powerful Trump critic, dies at 68 House Oversight panel demands DeVos turn over personal email records MORE (R-Utah) and ranking member Elijah Cummings (D-Md.) as well as Rep. Will Hurd (R-Texas) and Rep. Robin Kelly (D-Ill.), were sent to 24 federal agencies in total, including many not traditionally viewed as having law enforcement functions, like the Social Security Administration and the National Science Foundation.

Lawmakers have asked the agencies for documents and information related to their potential use of the devices.

The letters are indicative of a growing concern about the scope of the government’s use of the devices.

Earlier this year, advocates raised concerns about Justice and Homeland Security’s use of the StingRays for surveillance. Both departments later created new policies that required agents to obtain warrants in many cases before using the devices.

But recent reports indicate more agencies may be using the technology. The Guardian reported earlier this month that the Internal Revenue Service had spent money to upgrade the devices, though it was unclear whether the agency has actually used them. Activists have also raised concerns that exemptions in the DHS and DOJ policies governing the technology are too broadly drawn.