Secret phone surveillance technology was probably implemented in 2024. DNC

A device capable of intercepting phone signals may have been deployed during the 2024 Democratic National Convention (DNC) in Chicago, WIRED has learned, raising critical questions about who authorized its use and for what purpose.

The device, known as a cell site simulator, was identified by the Electronic Frontier Foundation (EFF), a digital rights group, after analyzing wireless signal data collected by WIRED during the August event.

Cell site simulators mimic cell towers to intercept communications, indiscriminately collecting sensitive data such as call metadata, location information, and application traffic from all phones within range. Their use has drawn widespread criticism from privacy advocates and activists who argue that such technology could be used to secretly monitor protesters and suppress dissent.

The DNC convened amid widespread protests against Israel’s assault on Gaza. While accredited influencers attended exclusive yacht parties and VIP eventsthousands of demonstrators faced a increased presence of law enforcementincluding officers from the US Capitol Police, the Secret Service, Homeland Security Investigations, local sheriff’s departments and the Chicago Police Department.

Concerns about potential surveillance prompted WIRED to conduct a first-of-its-kind wireless study to investigate the deployment of cell site simulators. Reporters equipped with two rooted Android phones and Wi-Fi hotspots running detection software were used Rayhunter— a tool developed by the EFF to detect anomalies in the data associated with these devices. WIRED reporters monitored signals at protests and event venues in Chicago, gathering extensive data during the political convention.

Initial tests conducted during the DNC did not reveal conclusive evidence of cell site simulator activity. Months later, however, EFF technologists reanalyzed the raw data using improved detection methods. According to Cooper Quintin, senior technologist at EFF, the Rayhunter tool stores all interactions between devices and cell towers, allowing for deeper analysis as detection techniques evolve.

The breakthrough came when EFF technologists applied a new heuristic to investigate situations where cell towers request IMSI (International Mobile Subscriber Identity) numbers from devices. According to EFF’s analysis, on August 18 – the day before the convention officially began – a device carried by WIRED reporters en route to a hotel housing Democratic delegates from states in the US Midwest suddenly switched to a new tower. That tower asked for the device’s IMSI and then immediately disconnected, a sequence consistent with the operation of a cell site simulator.

“This is extremely suspicious behavior that normal towers don’t exhibit,” says Quintin. He notes that the EFF typically observes such patterns only during simulated and controlled attacks. “This is not 100 percent undisputed truth, but it is strong evidence to suggest that a cell site simulator was deployed. We don’t know who is responsible – it could be the US government, foreign actors or another entity.

Under Illinois law, law enforcement must obtain a warrant to deploy simulators at cell sites. Likewise, federal agents — including those from the Department of Homeland Security — are required to secure warrants unless there is an immediate threat to national security. However, the 2023 DHS Inspector General report found that both the Secret Service and Homeland Security Investigations did not always meet these requirements.