The group capable of doing such things is FAR larger than you might believe. When I was a kid, I used to go to the Norwalk Public Library and pull both volumes of "The Chemistry and Technology of Explosives" from the reference shelf. Published in Krakow, Poland, IIRC. I had a copy of the Anarchist's Cookbook in the mid-70's. Found a second-hand store near my mom's place in Homeland, CA, that sold copies of the U.S. Army TM on improvised explosives. They had at least hundreds of them, priced at $3 each, about 10 or 11 years ago.
All of those publications have resulted in more people being hurt and killed during a construction attempt, than by successful deployments.