Reckoning, will be right back. This is a special edition of nightline, a mothers reckoning. Good evening and thank you for join us. Tonight the powerful and untold story of the mother of one of the columbine shooters. Nearly two decades later, sue klebold is speaking out. The mother trapped in contradiction between a little boy and all she believed he could be a a the murderer he became. And especially for the families of the victims, those urgent and anguished questions never answered, until now. Constitute massacre have been prevented, and warning signs heres abcs diane sawyer. Reporter it is 1999. A shock wave hits america. Two High School Boys in trench coats carrying shotguns, a semiautomatic weapon, and homemade bombs walk into their school and begin the slaughter of their classmates. Who were sitting on the grass eating their lunch. Who were hiding under tables. No defense from the terror. 13 are kill the. 24 are wounded. And we are all watching for the first time children run o