WordNet (r) 3.0 (2006):

law enforcement agency
    n 1: an agency responsible for insuring obedience to the laws

Moby Thesaurus II by Grady Ward, 1.0:

31 Moby Thesaurus words for "law enforcement agency":
   Cheka, FBI, Gestapo, Interpol, MP, MVD, Mounties, NKVD, OGPU, RCMP,
   SP, Scotland Yard, constabulary, county police, highway patrol,
   military police, police, police force, political police, posse,
   provincial police, riot police, secret police, security force,
   shore patrol, special police, state police, tactical police,
   troopers, vigilance committee, vigilantes