What drives a serial killer — and how did Rex Heuermann go undetected for so long? After pleading guilty to multiple murders tied to the Gilgo Beach case, Heuermann is now expected to speak with the FBI’s Behavioral Analysis Unit. Experts say the interviews could offer rare insight into how serial killers think, operate and evade detection. Psychologists and investigators explain how Heuermann allegedly lived a double life, concealed his crimes for decades and may have kept mementos from victims. They also discuss what the FBI hopes to learn — and how it could help solve future crimes. Sentencing is scheduled for June 17.