The Electronic Frontier Foundation has concluded that someone likely sent a mobile phone surveillance system during the Democratic National Convention last summer, according to a new report from Wired. The evidence for that statement comes from Cooper Quintin, a senior technologist at the EFF, who spent time investigating whether police technologies were deployed during the event from the event. Wired collaborated with the EFF to conduct an analysis of wireless signal data. What they found was evidence that someone had used a cell-site simulator to spy on the devices.

Cell-site simulators are controversial police devices that capture wireless signals from the air and store them for later analysis. Cell-site simulators basically perform Man-in-the-Middle style attacks, convincing mobile devices that they are cell towers and that they should send their signals to them. These attacks can reveal critical personal information, such as location data, call metadata, and app traffic, providing a critical window into mobile activity. A popular brand of cell-site simulators is the Stingray.

Wired reporters traveled to the DNC last summer and used phones with special software. That software was developed by EFF and is designed to capture data anomalies associated with devices. Wired describes their experiment like this:

WIRED attended citywide protests, events at the United Center (where the DNC took place), and social gatherings with lobbyists, political figures, and influencers. We spent time walking the perimeter along the march routes and through the planned protest sites before, during, and after these events.

In the process we get Bluetooth, Wi-Fi, and cellular signals. We analyzed the signals looking for specific hardware identifiers and other suspicious signs that might indicate the presence of a cell-site simulator.

After analyzing data from the devices, Quinton told Wired that it appears someone may have deployed a cell-site simulator in the area during the convention. Wired wrote that one of the devices brought by the reporters was “suddenly moved to the new tower.” That tower then “requests the device’s IMSI (international mobile subscriber identity) and then disconnects—a sequence consistent with the operation of a cell-site simulator.”

“It’s a very suspicious behavior that normal towers don’t show,” Quintin told Wired, about the analysis. “It’s not 100 percent indisputable fact, but it’s strong evidence to suggest that a cell-site simulator was deployed. We don’t know who’s responsible — it could be the US government, foreign actors, or other entities.

Gizmodo has reached out to the EFF for more information.

It is not known what would motivate someone to use a surveillance system at the Democratic National Convention, although there is an obvious reason why the police would want to monitor local telephones during . The convention was spoiled by ongoing protests because of the Biden administration’s support for Israel’s attack on Gaza. During the protests, above 40,000 Palestinians were reported killed, most of them women and childrenaccording to a UN estimate. Thousands of protesters gathered outside the DNC in Chicago. In some cases, protesters were arrested for breaching the barricade outside the convention center.