There's an extraordinarily fine line between agricultural fertilizers and industrial explosives.

I was thinking more along the lines of chemical weapons.

Same thing. Chlorine gas is commonly used in many industrial processes, but it's also a highly toxic chemical that has been used (and banned!) in warfare.

The biggest problem is so-called "dual use" chemicals and compounds where there are legitimate military and non-military uses.

You can build extremely dangerous explosives from chemicals found on the shelf at a drug store, but that doesn't mean that company is in the business of selling explosives.