Funny (but not funny, as it was a tragedy) that you should bring up the OKC bombings as an example of building a good bomb. McVeigh’s bomb didn’t work properly and failed to fully high order, it was just so much ANFO that it didn’t even matter.
They could very likely come up with something very functional, making IEDs, and even cooking your own explosive fillers is exceptionally easy, you really only need a very basic understanding of chemistry and circuitry (like grade 9 or 10 level). If extremists hiding in caves in Afghanistan could make consistent and effective devices with HME, then Joe Redneck with Internet access and over the counter access to Ammonium Nitrate Aluminum (Tannerite, which is legal in most states and often sold at gun stores) can probably do it too.
Oh absolutely, which is the worrisome part of all of this to me. Someone doesn’t even need to build something good, just good enough.