Bureau of Alcohol Tobacco and Firearms (ATF)

Department of Homeland Security (DHS)

Drug Enforcement Agency (DEA)

Federal Bureau of Investigation (FBI)

U.S. Immigration and Custom Enforcement (ICE)

U.S. Marshal Service (USMS)

U.S. Postal Inspection Service (USPS)

U.S. Secret Service

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For a more complete list see:  http://en.wikipedia.org/wiki/Federal_law_enforcement_in_the_United_States