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’s motives, and his life, remain largely a mystery. 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. Nowhere is that conversation more intense than in Newtown, where FRONTLINE finds a town divided and explores how those closest to the tragedy are wrestling with our nation’s gun culture and laws.