WordNet (r) 3.0 (2006):

FBI
    n 1: a federal law enforcement agency that is the principal
         investigative arm of the Department of Justice [syn:
         Federal Bureau of Investigation, FBI]

Moby Thesaurus II by Grady Ward, 1.0:

103 Moby Thesaurus words for "FBI":
   AEC, Appalachian Regional Commission, Atomic Energy Commission,
   Bow Street runner, CAB, CIA, Central Intelligence Agency, Cheka,
   Civil Aeronautics Board, District of Columbia, Domestic Council,
   EDA, EEOC, EIB, EPA, Environmental Protection Agency, FAA,
   FBI agent, FCA, FCC, FDA, FPC, FTC, Federal,
   Federal Aviation Agency, Federal Communications Commission,
   Federal Maritime Commission, Federal Power Commission, G-man, GAO,
   GSA, General Accounting Office, General Services Administration,
   Gestapo, Government Printing Office, ICC, Indian Claims Commission,
   Interpol, Interstate Commerce Commission, Library of Congress, MP,
   MVD, Mounties, NASA, NKVD, NLRB, NSF, National Mediation Board,
   National Science Foundation, National Security Council, OEO, OGPU,
   Panama Canal Company, RCMP, Railroad Retirement Board, SEC, SP,
   Scotland Yard, Secret Service, Sherlock Holmes,
   Smithsonian Institution, T-man, TVA, Tennessee Valley Authority,
   USIA, VA, constabulary, county police, detective, fed,
   highway patrol, hotel detective, house detective, house dick,
   inquiry agent, investigator, law enforcement agency,
   military police, narc, operative, plainclothesman, police,
   police detective, police force, political police, posse,
   private detective, private investigator, provincial police,
   revenuer, riot police, secret police, security force, shore patrol,
   sleuth, special police, state police, store detective,
   tactical police, treasury agent, troopers, vigilance committee,
   vigilantes