nightmare. >> translator: the day we are revenging because you started the assault. >> paris, november 13th. as hospitals and morgues in paris are inundated with the injured and the dead, the investigation began into how the attacks happened but there was little doubt about who was responsible. >> isis claimed responsibility very quickly. the french government within 24 hours identified isis as being responsible. >> reporter: quickly, investigators learn the identities and movements of the men who killed and died that night in paris. >> many had long histories of jihadist terrorist activity. some of the bombers in the paris attack were not on the 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 nationals and most had travelled to syria. several were known to both european and u.s. intelligence. >> the french say that for every one militant they are trying to follow, they need 25 people to monitor that person. so if you've got thousands of people, do the math. >> do the math and then ask the hard question, could what