Add the fact that it is happening around the DC area. Also the consistent reports of two men. The phrasing of the note on the tarot card "Mr. Policeman". The timing in relation to the recent announcement of attacks by Al zahwiri or whatever his name is.....And the deliberate movement from place to place. I think Al Queda is a really good guess.
If these jokers are not caught soon, they will either go to ground or they will move to NYC or Boston.
Do you know how many notes/letters were supposedly sent by Jack the Ripper during the search for him? Most, if not all, were hoaxes.
The idea of this being al Queda is a nice story. But I'd include in with a boat load of other theories.