Weapons of Mass Destruction. In particular, Sarin nerve gas. The chemical synthesis is second-semester organic chemistry class stuff, except for the protections required. Any pesticide plant could produce it in tank car lots. Some pesticides, parathion in particular, are pretty nasty themselves.
As to "what to do about it", here's a brief article by someone with training. The only criticisims that I've seen have been in the class of "That won't work if the Bad Guys have US chem/bio delivery systems". They don't. Unless, of course we sold it to them.