None Shall Sleep by Ellie Marney

In 1982, two teenagers—serial killer survivor Emma Lewis and US Marshal candidate Travis Bell—are recruited by the FBI to interview convicted juvenile killers and provide insight and advice on cold cases. From the start, Emma and Travis develop a quick friendship, gaining information from juvenile murderers that even the FBI can't crack. But when the team is called in to give advice on an active case—a serial killer who exclusively hunts teenagers—things begin to unravel. Working against the clock, they must turn to one of the country's most notorious incarcerated murderers for help: teenage sociopath Simon Gutmunsson. Despite Travis's objections, Emma becomes the conduit between Simon and the FBI team. But while Simon seems to be giving them the information they need to save lives, he's an expert manipulator playing a very long game...and he has his sights set on Emma.