I've worked with cyanide quite a bit in the gold mining industry so I thought I'd throw out a few facts about the stuff. First of all, it's the mixture of sodium cyanide (or potassium cyanide or calcium cyanide) with acid that is dangerous. Cyanide in acid releases cyanide gas: HCN. This is quite deadly - there's a reason they used it in gas chambers. The reaction will work with any acid; nitric, hydrochloric, acetic or whatever would all be effective.

I would say, though, that it would be impossible for these militia loons to make an effective 'cyanide bomb'. Both the Axis and Allied sides tried to develop one in WW1 with no success. They used chlorine and mustard gases in the trench battles for that reason. The technical problems with a 'cyanide bomb' remain today. I couldn't build one and I suspect I know a great deal more about the technology than these people.

Cyanide could be used with terrible effect in an enclosed space, as gas chambers prove, so these people are still very dangerous. I can easily imagine scenarios where cyanide gas could be introduced into, say, an office air supply. Sodium cyanide is also very soluble so it is a threat to water systems, without the use of acid. It breaks down quickly in the presence of air and sunlight so they'd need to get into a piping system for that to be effective. The chlorine in water supplies would tend to break it down too but probably not very effectively.