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 set up to deal with the long-term threat they posed. >> (translated): that was our tragedy, in a way. we thought that if there are no attacks on french soil, then the anti-terrorist system is working correctly, therefore everything is fine. in fact, nothing was fine. these may have been terrorists linked to al qaeda, but they