DC buses are policed by DC Transit Police.
The streets of DC, however, are policed by DC Metropolitan Police.
The 1600 block is where the White House is, and falls under the the Secret Service.
The area between the 1600 block and the 900 block goes near several areas designated as US Parkland, and therefore is under jursdiction of the US Park Police.
The FBI Headquarters is on the 900 Block of Penn. Ave.
Any "terrorist-related" event occurring on transit in the US may fall under TSA and DHS jurisdiction.
If the firearm is owned by someone who is not a DC resident, BATFE may have jurisdiction.
Ultimately, in DC, it depends on which LEA responds first, and which LEA decided it WANTS jurisdiction. Usually, events like this get handed to DCMP or DCTP, unless they actually occur on Federal property, or directly involve a Federal Official.
In other words, jurisdictional authority in DC is a crazy-quilt patchwork of "not my responsibility"... Unless it's a high-profile case that an agency can get political traction from...
So pretty much ANY LEA you could name in DC would be the right answer... Or the wrong one...
And, BTW, carrying a concealed firearm in DC without a license is a FELONY, and carries penalties of up to 5 years in prison and up to $10,000 in fines, and that would most likely be ON TOP OF the charge of possessing an unregistered firearm, which is a misdemeanor carrying up to a 1 year/$1000 penalty. And remember that in DC, your ammo must also be registered, and EVERY round that is not legally possessed is a FELONY...
http://www.criminaldefenselawyer.co...ms/district-columbia-concealed-weapon-charges
http://mpdc.dc.gov/mpdc/cwp/view,a,1237,q,567003.asp#7