. >> narrator: louis caprioli was then the counterterror chief of french domestic intelligence. rotella first met him while reporting on the buttes-chaumont gang. >> (translated): french domestic intelligence and the police were watching that group. in january 2005, the authorities began to dismantle the network. cherif kouachi was arrested as he was about to board a plane to go to iraq and fight. >> narrator: kouachi was sent to fleury-mérogis prison to await trial. while he was locked up, his extremist connections only deepened. the prison was a hotbed of jihadism dominated by al qaeda veterans. kouachi became friends with another radicalized criminal who would ultimately join him in his terrorist project: amedy coulibaly. >> (translated): they could communicate with each other. it was totally porous. it was disturbingly easy to form connections. so they expanded their network and became worse than when they arrived in prison. >> narrator: marc trévidic was a top counterterror prosecutor and judge who investigated kouachi's network. he says the french judicial system was not s

for the former counterterror chief louis caprioli, it was unsettling news. >> (translated): it quickly became known it was the kouachis. that rang a bell immediately. i thought, "yes, these are the people from 2005." so immediately i thought that there might have been a failure somewhere. when an attack happens and you had no prior intelligence, that's one thing. but when you find out that you could have prevented it, that is a tragedy. >> narrator: after a massive manhunt, the kouachi brothers were killed in a shootout with french police. (gunfire) they declared allegiance to al qaeda shortly before they died. that same day, their friend amedy coulibaly carried out his part of the plot, shooting four people dead in a jewish supermarket before being killed by a swat team. although he had been radicalized by al qaeda, he claimed allegiance to isis. >> (translated): i was surprised. at first i couldn't understand this alliance between coulibaly from isis. and kouachi of al qaeda. how was the association between al qaeda and isis even possible? i realized it's simply a question of relatio