Officials with the Department of Homeland Security detected potential surveillance activity near “sensitive facilities” in Washington, including the White House, according to a study conducted last year.

Officials disclosed the activity, associated with devices commonly known as “Stingrays,” in a letter to Sen. Ron Wyden Ronald (Ron) Lee WydenGOP set to release controversial Biden report Democrats fear Russia interference could spoil bid to retake Senate GOP senator blocks Schumer resolution aimed at Biden probe as tensions run high MORE (D-Ore.), which was first reported by The Washington Post this week.

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The revelation boosts long-held suspicions that foreign actors are using the technology to conduct spying in the nation’s capital.

Christopher Krebs, the acting head of the National Protection and Programs Directorate (NPPD), Homeland Security’s cybersecurity unit, explained in the May 22 letter that the department initiated a “limited pilot project” in the D.C. region last year to understand the activity of “Stingray” devices — formally known as international mobile subscriber identity (IMSI) catcher technology.

An IMSI is a unique identification number that is used to recognize any one mobile device on a cellular network. IMSI catcher technology mimics legitimate cellphone towers in order to intercept cellular communications, allowing for eavesdropping.

The tracking devices are often referred to as “stingrays” after the StingRay brand widely used by state and local police officers.

In the letter, Krebs acknowledged that Homeland Security observed “anomalous activity that appeared consistent with IMSI catcher technology within the [U.S. Capitol Region], including locations in proximity to potentially sensitive facilities like the White House” when conducting the analysis between January and November of last year.

However, the official noted that NPPD “has neither validated nor attributed such activity to specific entities, devices, or purposes.”

“It is my understanding that relevant law enforcement and counterintelligence agencies conducted further investigation and determined some detected signals were emanating from legitimate cell towers,” Krebs wrote.

The new detail comes after Homeland Security acknowledged in an earlier letter to Wyden that officials had observed likely IMSI catcher technology activity in the D.C. region. It represents the first instance of U.S. officials acknowledging such activity near the White House.

Wyden and others have clamored for more information from Homeland Security on the possible surveillance activity. Sens. Cory Gardner Cory Scott GardnerOvernight Health Care: US coronavirus deaths hit 200,000 | Ginsburg's death puts future of ObamaCare at risk | Federal panel delays vote on initial COVID-19 vaccine distribution The Hill's Campaign Report: GOP set to ask SCOTUS to limit mail-in voting Senate GOP sees early Supreme Court vote as political booster shot MORE (R-Colo.), Rand Paul Randal (Rand) Howard PaulSecond GOP senator to quarantine after exposure to coronavirus GOP senator to quarantine after coronavirus exposure The Hill's Morning Report - Sponsored by National Industries for the Blind - Trump seeks to flip 'Rage' narrative; Dems block COVID-19 bill MORE (R-Ky.), and Ed Markey Edward (Ed) John MarkeySchumer: 'Nothing is off the table' if GOP moves forward with Ginsburg replacement Democrats see fundraising spike following Ginsburg death Democratic senator calls for eliminating filibuster, expanding Supreme Court if GOP fills vacancy MORE (D-Mass.) joined Wyden in calling for the department to release an unclassified PowerPoint presentation on the activity in April.

Krebs indicated in his latest letter that the department would not do so, describing the presentation as for official use only and therefore “not appropriate” for public release.

Separately, Krebs also acknowledged that the department has received reports of “nefarious actors” possibly exploiting security flaws in Signaling System Seven (SS7), an international system that connects mobile phone networks, in order to “target the communications of American citizens.” He did not expand further on those reports.

In a statement Friday, Wyden called on the Federal Communications Commission and telephone companies to do more to protect Americans’ communications. He accused the Trump administration and private phone companies of doing a “terrible job” of protecting Americans from being surveilled or tracked.

--Updated at 1:12 p.m.