thousands of videos are strewn across the internet. much of it, of course, is sickeningly violent, unbearable to watch. the awful beheadings, the fiery murders. but all of it may add up to the single biggest reason for the success of isis. like so much of what the group does, this is a terror tactic we have not seen before. and it is frighteningly effective. >> 20 years ago, you could never find the three people in minnesota who would be attracted to the isis ideology. today you can and they can find you. isis has used facebook, twitter, google, and the worldwide web as its command and control system. >> the violence in isis propaganda is enhanced by artful editing, special effects, and powerful music. ♪