Things don't fall between cracks, they fall through cracks. A minor point.
Sure the red flags were up, but so were so many others raised by the left, such as killer suvs, cigarettes, accusations of republicans wanting to poison the air and kill children and grandpeople, result-free soccer leagues, and all sorts of nonsense... We have/had several congressmen who are poll driven and unless the matter of terrorist action upon the US was on the vote-radar then little would have been done about it. The left deserves an amount of blame for this mess, supporting a president who put himself above all others among other things. It's no wonder we didn't see it coming.