After his arrest, the boy's mother was stunned to discover that her 12-year-old had been learning how to kill and gorging on videos of decapitation and torture so gruesome they made even case-hardened French court officials look away. The mother told criminal investigators that she'd thought her son had been playing video games and doing homework during the hours he spent in his room. From there, more searching, automated algorithms that steer users' online experiences and the boy's curiosity ultimately led him to encrypted chats and ultraviolent propaganda pumped out by Islamic State militants and other extremist groups that are worming their way via apps, video gaming and social media into the minds of the very young.