ready to go back to the streets. >> i'm john douglas. for close to 25 years, i worked for and ran the fbi's behavioral science unit. as a criminal profiler, my job was to help crime investigations across the country, by examining evidence for signs that could help point the way to the killer. the only way to truly understand the criminal mind is to get inside it. i did that by going to prisons around the country, like this one, going head-to-head with the nation's most violent offenders. in my fbi unit we met with people like john wayne gacy, who killed dozens of teenage boys and buried them in his basement. and joseph paul franklin, a violent drifter who targeted interracial couples. every one of these guys has something to teach us, and what i learned from them has helped put other serial killers away. in just a few moments, you'll meet a man who embodies every parent's worst nightmare. he preys on friend's children,