A U.S. Secret Service vehicle outside the White House in Washington, D.C.

The Department of Homeland Security last year found evidence of devices that can secretly catch cell phone communications around the White House and other "potentially sensitive" areas of Washington, D.C., a letter made public Friday reveals.

Sen. Ron Wyden, D-Ore., said that the letter, written to him by Homeland Security official Christopher Krebs, is "more evidence" that Americans are "being spied on, tracked, or scammed," possibly by foreign spy agencies in some cases.

Wyden said phone companies and the Federal Communications Commission should be taking action to strengthen cell phone security on the heels of the letter.

Homeland Security said sensors it deployed from January 2017 through November spotted activity that appeared consistent with the devices, which can monitor individual cellphone calls and texts. Known formally as International Mobile Security Identity devices, they are commonly known as StingRays.

Such devices are known to be used by foreign spies.

But the department said it had not validated the findings or attributed them to "specific entities, devices or purposes."