FED.US is available for use by agencies of the U.S. Federal government. Most government agencies use .GOV but some do operate under FED.US. The particular agency would be a subdomain under this second-level.

The following are the locality-based top-level domains under .US:
AL.US Alabama — AK.US Alaska — AZ.US Arizona — AR.US Arkansas — CA.US California — CO.US Colorado — CT.US Connecticut — DE.US Delaware — DC.US District of Columbia — FL.US Florida — GA.US Georgia — HI.US Hawaii — ID.US Idaho — IL.US Illinois — IN.US Indiana — IA.US Iowa — KS.US Kansas — KY.US Kentucky — LA.US Louisiana — ME.US Maine — MD.US Maryland — MA.US Massachusetts — MI.US Michigan — MN.US Minnesota — MS.US Mississippi — MO.US Missouri — MT.US Montana — NE.US Nebraska — NV.US Nevada — NH.US New Hampshire — NJ.US New Jersey — NM.US New Mexico — NY.US New York — NC.US North Carolina — ND.US North Dakota — OH.US Ohio — OK.US Oklahoma — OR.US Oregon — PA.US Pennsylvania — RI.US Rhode Island — SC.US South Carolina — SD.US South Dakota — TN.US Tennessee — TX.US Texas — UT.US Utah — VT.US Vermont — VA.US Virginia — WA.US Washington — WV.US West Virginia — WI.US Wisconsin — WY.US Wyoming — PR.US Puerto Rico — VI.US U.S. Virgin Islands

The following are non-locality-based domains under .US:

• NSN.US - Indian tribes ("Native Sovereign Nations")
• FED.US - Federal Government (typically uses .GOV)

You may also be interested in:

• .GOV - Federal, State and Local government agencies in the United States
• .MIL - U.S. Military departments and agencies