The IRS is the latest federal agency to have been found with Stingray surveillance technology. The Guardian has reported on documents that show the IRS had access to the devices, though the scope of their use remains unknown. A heavily redacted invoice, obtained through a Freedom of Information Act request, shows that the IRS paid communications and defense contractor Harris Corporation around $65,000 to upgrade a Stingray in 2012, plus around $6,000 for training. It made other purchases between 2009 and 2012, but the details remain unknown.

Stingrays, also known as cell-site simulators, gather cellphone data by imitating a cell tower. They've proved attractive to both local law enforcement and federal agencies; besides the IRS, at least 12 other agencies — which range from US military branches to the Drug Enforcement Administration — have access to them. But privacy advocates have complained that they take a dragnet approach to collecting information, and they're often used with relatively little oversight or transparency.

Recently, federal agencies have taken steps to increase the transparency around cell-site simulators and raise the legal bar for their use. In September, the Justice Department pledged to seek official warrants before using the technology, and the Department of Homeland Security followed last week.