[ErrantX]

I suspect there is a missing qualifier there. i.e; "Bombs are easy to make if you have some common sense".

Bombs can come in all shapes and forms; I'm willing to bet the vast majority of people here (for example) could overcome the problem of a) building it and b) not using obvious "tell tale" materials. Off hand I recall Meth can be created out of "household" products and the reactions involved can be very volatile/explosive - that seems like the possible makings of an effective bomb right there.

(also; tracking fertilizer sales is probably not going to help you ID terrorists :) I bet the numbers are just crazy - also, any sensible terrorist will be nicking small quantities from farms etc.)

On the other hand terrorist converts acting alone are, I suggest, less likely to be able to do the correct research and construction.

[danudey]

Somehow I think setting up a meth lab in a parkade near a building's support structures would be noticed by security.

[ErrantX]

You don't have to set up a lab; the bomb is produced by unstable chemicals. At the very least you could drive up a van and mix those chemicals together.