>> horrified screams coming from inside theater. >> each time we meet the face of evil we ask how, why, young men, boys, turned into butchers, maiming and murdering hundreds of innocents. striking at the heart of one of the world's great cities. blowing a plane out of the sky, entire families turn to dust. all of it done with the reach and precision we have not imagined isis capable of. as the world cries out for vengeance, perhaps the most important question is this, how could we not have known? ♪ the answer is, we did. this is the story of what we knew about isis and when we knew it.