but there was little doubt about who was responsible. >> isis claimed responsibility very quickly. the french government within 24 hours had identified isis as being responsible. >> quickly the investigators would learn identities and movements of the men who killed and died that night in paris. >> long histories of jihadist terrorist activities. some of the bombers in the paris attack were not on u.s. watch lists. some of them were. because some of them were quite well known jihadi terrorists. >> almost all of the attackers were european nationalists and most had traveled to syria. several were known to both european and u.s. intelligence. >> the french say for every one militant they're trying to flower, they need 25 people to monitor that person. if you've got thousands of people, do the math.