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

the last three years, they had been monitoring al qaeda veterans of the buttes-chaumont gang: cherif kouachi, and his brother said. meanwhile, kouachi's old associate amedy coulibaly had just been released from prison. french domestic intelligence now decided to stop watching them and shift surveillance resources onto the growing threat from isis. >> (translated): for more than three years the surveillance and wiretappings had yielded nothing, so it was stopped at that time. this was mid-2014, which means that for six months, the services didn't know what the kouachi brothers were up to, and didn't know about the alliance they developed with amedy coulibaly, who had never been monitored since he stepped out of prison. (man shouting) (gunfire) >> breaking news out of france. the french police right now e hunting for masked gunmen who stormed the offices of the satirical nepaper, opened fire in the french capital today. at least 12 people are dead, four others are in critical condition. >> (translated): charlie hebdo was a turning point. the concept of attacking in the heart of europe was a ne