Org. once headed by Robert Mueller (3)

I believe the answer is:

fbi

(Other definitions for fbi that I've seen before include "law enforcers" , "US law enforcement agency" , "Initially, J Edgar Hoover's law force" , "J. Edgar Hoover's organisation" , "J. Edgar Hoover long its director" .)

I've seen this clue in The Wall Street Journal.

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