The Counterintelligence Division (CD) is a division of the National Security Branch of the Federal Bureau of Investigation. The division protects the United States against foreign intelligence operations and espionage. It accomplishes its mission of hunting spies and preventing espionage through the use of investigation and interaction with local law enforcement and other members of the United States Intelligence Community. In the wake of the September 11, 2001 attacks, the division's funding and manpower have significantly increased.
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en.wikipedia.org/wiki/FBI_Counterintelligence_Division