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It may or may not be somewhat common knowledge at this point since it's been covered numerous times in popular media, but the Bureau of Prohibition was part of the Department of the Treasury.  (More specifically, it was initially part of the IRS, becoming its own bureau within the Treasury Department by 1927.)
During the span of Prohibition, it employed between about 1,500 and 2,500 shabbily-paid agents to investigate illicit alcohol trade and enforce the 18th Amendment across the nation.  (It was later moved to the Justice Department, then to the FBI.  It exists in a transmogrified capacity as the ATF these days.)