If you ever take a drive around DC and keep your eyes open, you will notice that dozens of different agencies with armed agents and 'police' style vehicles.
-FBI, BATFE, DEA, US Marshals
-Department of Education
-Secret service uniformed and nonuniformed, BEP, IRS
-US Capitol Police
-DoD, DIA, etc
All you have to do is spend a day driving around DC and you will see different ones everywhere. Heck, I think I've even seen FDA police. Most of the time all these officers in the District appear to be tasked with protecting their respective agency buildings.