paris under attack by isis gunmen and bombers. some of them french citizens. several of the attackers and the alleged mastermind abdelhamid abaaoud were previously known to european and u.s. intelligence. >> how did they miss an attack of this scale? >> the sheer scale of radicalization in europe is like nothing they've ever dealt with before. we're talking about tens of thousands of people who've become radicalized in some way, shape, or form. so there are just too many people for them to watch intensively. >> not that many takers for the isis ideology here in the united states compared to what you're seeing in europe. >> peter burgen co-authored the new report "isis in the west: the new faces of extremism." those that are recruited in america are young and active