NOT TO STEREOTYPE OR ANYTHING, but it seems that arabs/muslims generally like for stuff to blow up real big and have lots of wounded victims for psychological effect, especially women and kids.
Also, consider this: Tim McVeigh and Terry Nichols took an afternoon off from whatever their day jobs were, stirred up some fertilizer and diesel fuel from some plans they got off the internet and blew up a whole federal building, right?
If it's that damn easy, why wouldn't terrorists do that instead of going to all the trouble of shooting individual people and taking the risk of getting caught at it? (/end sarcasm)