If you take into consideration the fact that the US Coast Guard is charged with enforcing the nations laws, then it would be THAT agency which began its life as the US Revenue Cutter Service.
The oldest federal agency in the U.S.A is the U.S. Marshals Office Provided by Jamason Wells W.T.I. Florida
usps
The Environmental Protection Agency.
That would be FEMA which stands for the Federal Emergency Management Agency.
Where? In the US its the IRS.
employee (US Federal worker and/or manager)
Department of Defense.
FDA
CIA
Yes, it is.
yes it is... it stands for the Federal Bureau of Investigation