My dad has a lovely Belgium made Browning .22 pump action (looks like a miniturised pump action shotgun). A very quiet, accurate rifle. He also had a friend who had a .22 bolt action fitted with a scope and a silencer, once again very accurate.
The scoped silenced .22 was nasty. It was small, very portable, and the low velocity (subsonic) rounds did a lot of damage (a lower velocity round tends to do more damage if you hit them right). Both my father and this guy were involved in the security business and got a lot of international publications (corporate, not the stuff you pick up off the shelf) and most of the security briefs were about .22 calibre weapons being the biggest dangers.
I got to play with M-16s in air cadets for a bit, and SLR's. The SLR L1A1 was like a small cannon. The M-16 (.223) wasn't much louder than a .22LR.
I'd agree its either someone covering up a homocide, or Al Qaeda testing a new method of terrorism out.
If its the latter, $5 says the DC shottings stop and this guy reappears somewhere on the other side of the states a month later.