My second guess would be a little leave behind gift from a CIA Predator strike.
Respectfully I say, there's no chance. I can tell you as a former Marine with twenty, the US government is deadly serious when it come to the deployment of chemical or biological agents. In short, we don't do it - EVER.
I've never met an officer that would load a biological agent into a delivery vehicle - it is against every rule of war and our people don't break the rules of war, contrary to what Keith Olbermann et.al. believe.
that is just too silly to say for so many reasons