As Immigration and Customs Enforcement steps up its deportation efforts, the agency is turning to a controversial surveillance device. According to a report by The Detroit News, local agents recently used a cell-site simulator (also known as a Stingray) to locate a Salvadorean restaurant worker, tracing his cell-phone signal to a home in the Detroit metro area. It’s one of the first cases of ICE using Stingrays under the Trump administration, raising new questions about the federal use of the device for civil immigration enforcement.

Typically used to locate devices tied to a specific phone number, cell-site simulators have been the subject of significant controversy in recent years. The devices work by mimicking the signal of a cell tower, then collecting information from every device that attempts to connect. As a result, they can disrupt cell service in areas where they are used, and often collect vast amounts of information from non-targeted phones.

Use of the devices is widespread within law enforcement, but remained secret for many years. The devices were only made public after a protracted legal appeal resulting from a fraud case. Among other projects, the US Marshals service deployed the devices from small, low-flying planes as a way of locating a single fugitive in a dense urban area.

The Department of Homeland Security (which includes ICE) operates at least 124 Stingray devices, according to a congressional report last year. In 2015, DHS issued an agency-wide policy requiring a search warrant to deploy the devices.

ICE has arrested 41,300 people for deportation since Trump’s inauguration, according to recently released statistics. More than 10,000 of those people had no criminal conviction, a sign of the agency’s new focus on available targets rather than criminal offenders. Despite the surge of arrests, deportations were actually down compared with the same period last year as a result of the growing backlog in immigration courts.