In the wake of the mass killings at Sandy Hook Elementary School, FRONTLINE investigates a young man and the town he changed forever. Adam Lanza left behind a trail of death and destruction, but little else. He left no known friends, no diary. He destroyed his computer and any evidence it might have provided. His motives, and his life, remain largely a mystery. In collaboration with The Hartford Courant, FRONTLINE looks for answers to the central–and so far elusive–question: who was Adam Lanza?