There are many Federal Agencies who hire people with degrees to become investigators.
Among these are the DEA, FBI, ATF, Secret Service, and ICE.
The CIA and NSA are not Law Enforcement Agencies. They work in intelligence, which is related to, but different from Law Enforcement. Consequently, there are no investigative or "detective" positions at those agencies, though they have analysts to go over the intel.
The other option you would have is to apply at a local or state agency, learn Law Enforcement from the patrol position (a very good way to do it) and then put in for a spot as a detective when one becomes open after you have been on 3-5 years (the minimum time necessary for promotion or transfer).
Note that there are uniform requirements for many of these agencies, and I am unfamiliar with any Federal, State, or Local Agencies that employ a sworn officer or investigator with those garment requirements.
There will be several Federal Agents along soon, I am sure, who will be able to shed more light on this at the Federal Level, with which I am not as familiar.
-Citicop.
Sometimes there's Justice...
and sometimes, there's Just Us
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In memory of DCLaw- EOW@RealPolice 02-20-2007.
We won't rest 'till we find the mutt.