Washington (CNN) Cellphone surveillance devices were detected near the White House and other sensitive locations in the Washington, DC, area last year, a government study found.

The spying technology, called International Mobile Subscriber Identity devices and known alternatively as Stingrays or IMSI catchers, was discovered as part of a review by the Department of Homeland Security conducted last year and detailed in a letter the agency sent to Sen. Ron Wyden, D-Oregon, last month.

While the presence of the cell-site simulators in the nation's capital had been previously reported, their detection near the White House first emerged in the May letter after elected officials pressed DHS for more information amid concerns about the potential national security threat posed by the devices and vulnerabilities in the nation's telecommunications network.

Writing to Wyden in May, DHS's acting National Protection and Programs Directorate head, Christopher Krebs, said the department had not determined which groups were behind the surveillance activity and raised the possibility that the signals detected could have come from legitimate sources.

In the 2017 study, DHS "did observe anomalous activity that appeared consistent with IMSI catcher technology within the [National Capitol Region], including locations in proximity to potentially sensitive facilities like the White House," but had "neither validated nor attributed such activity to specific entities, devices, or purposes," Krebs said.

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