The production and packaging might be difficult. Distribution and delivery, considering the rather "perfect" team effort we've learned of since 911, would be less difficult (IMHO).
Consider the water supply systems of lakes, rivers, et al; miles and miles of shoreline.
Consider city high-rises and wind - or hot air and weather balloons or a balcony in the atria of a mall.
Consider pin-pricks in a crowded walkway or subway.
Patrols? Guards? Manpower intensive!!
Whether there'll be one concentrated incident with a million casualties or a thousand wide-spread incidents of ten thousand casualties - lot's of places to do one's thing, particularly if you're a BG expecting to join your targets.
-Andy
http://ironbarr.com/militia.htm