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? Also this hour: In the aftermath of the tragedy, President Obama called for a national conversation about guns in America. Nowhere is that conversation more intense than in Newtown, where FRONTLINE finds a town divided and explores how those closest to the tragedy are now wrestling with our nation's gun culture and laws.