Intrusive devices called International Mobile Subscriber Identity (IMSI) catchers located in Washington, DC have been disclosed by the U.S. Department of Homeland Security after a memo from DHS official Christopher Krebs to Senator Ron Wyden (D-OR) was recently made public.
Memo: https://regmedia.co.uk/2018/04/03/wyden_response.pdf
DHS response: https://regmedia.co.uk/2018/04/03/wyden_enclosure.pdf
The installers of these catchers disguised them as real phone masts (which sometimes mimic tall trees or other familiar objects) and reportedly tracked people's movements, while eavesdropping on their private calls and/or texts.
IMSI catcher capabilities are quite impressive; they can apparently intercept metadata and communications by forcefully downgrading a user's smartphone to 2G mode. Security experts have demonstrated these types of devices are able to "collect" both texts and phone calls. Emails and internet usage may also be attainable.
UK publication The Register called the mysterious tower operators "Uncle Sam's g-men" which could be a reference to possible deep state operatives.
Although these pretend-towers are sometimes used by both federal and local law enforcement to assist with gathering information on suspects, those who are knowledgeable about the technology have expressed concern over the possibility that even more sophisticated versions have been used to "spy" on foreigners and even United States citizens.
Government purchases of such devices are many times "shrouded in secrecy," according to the American Civil Liberties Union. That being said, they were able to create a map that shows which states and agencies own IMSI catchers, such as the 'Stingray':
