Three-letter Agencies 

Three Letter Agencies are a general category of "spook" agencies, which defend America, but generally act mysteriously. A knock on the door at any hour from any of them should make you want to run for the back door or jump out the nearest window, except it won't help since they're already watching it.

Some are likely to make you disappear. Some may just want to question you. Some, you'll never know about, but they're already watching you. It doesn't matter that you may not have heard of some of them, they already know who you are, who you've dated, and what you ate for breakfast.

ATF - Bureau of Alcohol, Tobacco, Firearms and Explosives

CIA - Central Intelligence Agency
DEA - Drug Enforcement Agency
DHS - Department of Homeland Security
DIA - Defense Intelligence Agency
DOD - Department of Defense
DOE - Department of Energy (yes, them too)
FBI - Federal Bureau of Investigation
INR - Bureau of Intelligence and Research
ISR - Air Force Intelligence, Surveillance and Reconnaissance Agency
NRO - National Reconnaissance Office
NGA - National Geospatial-Intelligence Agency
NSA - National Security Agency
ONI - Office of Naval Intelligence
TFI - Office of Terrorism and Financial Intelligence

USSS (United States Secret Service) falls into this category, even though it has 4 letter in its acronym.

{Phone Rings}

Friend 1) Hey, how's it going

Friend 2) Ok, but there's a black van parked in front of my

house. I think it's a three-letter agency and {static}{click}

Friend 1) Hello? Hello? What would one of the three-letter agencies want with you?

Friend 2 is never heard from again.

Three-letter Agencies by JWSmythe June 4, 2009

Three-letter Agencies 

The federal agencies comprised of three letters.

Usually refers to the ATF (bureau of Alcohol, Tobacco and Firearms), CIA (Central Intelligence Agency), DOE (Department of Energy), FBI (Federal Bureau of Investigations), IRS (Internal Revenue Service) and NSA (National Security Agency)

Three-letter Agencies by Larstait November 6, 2003