When someone says that the intelligence community really dropped the ball by not recognising the threat of this or that person in advance, ask them which threat is currently being ignored. The question is not whether some other "preventable" horror will befall us, but when and who will do it. Chances are there's someone out there right now who is giving off "all the telltale signs", but with the mountain of data to sift and any number of crazies who will never do anything dangerous, picking that needle out of the haystack is not easy.
Mokalus of Borg
PS - And reacting to every one of them is out of the question.
PPS - Nobody has that kind of time, money or energy.