Terrorist cell(s).
A caller to Chris Core's radio talk show on WMAL last night pointed out that the sniper started shooting the day after Ari Fleischer made his "single bullet" comment about killing Saddam. Since Chris Core then said he had thought of the same thing, I assume the caller was right on the chronology.
I believe this is all directed at our political leaders in Washington, and that the shootings, like the anthrax, are meant to deter an attack on Iraq.