The Central Intelligence Agency (CIA) is a civilian foreign intelligence agency of the United States government. It is an independent agency that operates under the direction of the Director of National Intelligence, who reports to the President of the United States. 

The Defense Intelligence Agency (DIA) is an agency within the United States Department of Defense (DoD) that serves as the primary producer and manager of military intelligence for the DoD and the U.S. Armed Forces. 

The Federal Bureau of Investigation (FBI) is the principal federal investigative agency of the United States. It operates under the jurisdiction of the Department of Justice and serves as both a domestic intelligence agency and a federal law enforcement agency. Here is an overview of the FBI.

The National Security Agency (NSA) is a United States intelligence agency responsible for gathering, analyzing, and protecting classified information and communications related to national security. It operates under the Department of Defense and is one of the 18 agencies within the U.S. intelligence community.