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... See More
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 and 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 that may have been missed. Here's ABC's Diane sawyer. 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 out of their school fleeing mayhem. A lot of students -- Coverage of the columbine -- Reporter: We see a wounded student struggling out the window of his high school to escape with his life. And for the years to come, we would all be asking the same questions -- who were these killers? And what kind of parents could produce children like these? Someone wasn't doing their job. Reporter: For 17 years now, the parents of Dylan klebold and Eric Harris have lived their secrets, unwilling to step forward. Until a gray-haired woman makes her way into a room. In the course of this day at one point we see her pacing. Perhaps grappling with her decision to step forward after 17 years hiding from cameras. Years of being hated, threatened, she says afraid and ashame the. So klebold is now 66 years old. For the families of 13 people who died and 24 people who were injured, most of them children, what is it you want to say to them? . The one thing, of course, that I want to say is, I am so sorry for what my son did. Yet I know that just saying I'm sorry is such an inadequate response to all this suffering. There is never a day that goes by where I don't think of the people that Dylan harmed. You use the word harmed. I think it's easier for me to say harmed than killed. It's still hard for me after all this time. Is that about a certain need to deny what happened? I don't know. Perhaps. Perhaps. Reporter: She seems to be trapped in a contradiction. Remembering the son she once had and the murderer he became. Called him the sunshine boy? We did when he was little, yeah. He had this sort of a mane of golden hair. It was just thick and round and -- he was such a happy, precocious, brilliant little child. For all the parent hot have said, I would have something, I would have just known. I know. Before columbine left-hand side, I would have been one of those parents. And I guess that's why certainly why I'm here talking to you today. Reporter: She has written a book called "A mother's reckoning." In it she says all the lessons of her regret which began on the day she woke up an ordinary wife and mother, fast forward 24 hours and I was the mother of a hate-crazed gunman. She remembers the shocking prayer she found herself praying after learning her son might be one of the columbine shooters and the shooting was still going on. The police were there. The helicopters were going over. And I remember thinking, if this is true, if Dylan is really hurting people, he has -- somehow he has to be stopped. And then at that moment I prayed that he would die. That god stopped this. Just make it stop. Don't let him hurt anybody. Reporter: But by the end of the day, her son and his friend would take the lives of 13 people and then take their own. In her book sue klebold writes, her son wasn't the pinwheel-eyed portrait of evil we know from cartoons. He was shy, like bible, hands-on parents, put them to bed with stories and prayers and hugs. Reporter: Sue klebold calls Dylan her shiny penny. In gifted classes. Loved little league. Gave big hugs and kisses. Built tall ships out of legos. He wouldn't just work on one puzzle, he'd dump them into a big mountain so he could solve five or six of them at the same time. Reporter: She says he was easily embarrassed, tearful and hard on himself if he made a mistake. Shy heading into adolescence. He talked about looking weird? He was a tall, gawky kid with glasses. Reporter: She says she does notice something, that Dylan seems to be losing interest in good grades. But she now says, as she looks back over her life, she is male a big mistake. Her son is changing. But she writes off the changes. As an adolescent phase. Sometimes he would seem distant or quiet. And I remember asking him, are you okay? Are you sure you're okay? You seem so tired. And he'd stand up and say, I've got a lot of homework, I need to go to bed. And you'd let it go. I let it go. And that's the difference. I would dig. If it were me today, I would dig and dig and dig. Reporter: She has no idea her 15-year-old son has begun a secret journal. And his first entry is this. Thinking of suicide. I hate my life, I want to die. I have a nice family, good house, couple of good friends. No girls. Nobody accepting me even though I want to be accepted. But at this point we want to be very clear -- 80% to 90% of depression can be treated. And even suicidal depression is not an explanation for a pathway to violence. Dr. Gregory Fritz wants to warn parents tonight that suicidal depression is real and can strike any teen anywhere. Somewhere between 15% and 20% of high school kids say that they have thought about suicide in the past year. Reporter: But more than 17 years ago, sue klebold says she knew so little about teen depression. Could you have prevented what happened at columbine? If I had recognized that Dylan was experiencing some real mental distress, he would not have been there. He would have gotten help. Reporter: In this video from his junior year he seems self-conscious but talks about the future, going to college. So you get better chances at high school as far as college goes, maybe a scholarship. Reporter: Around the same time, a series of troubling events. He hacks into the school computer system with some friends. They're all suspended for three days. He scratches an epithet on the locker of a kid he thinks is taunting him. Then the big shock, he and another kid break into a van, steal electronic equipment, and police make an arrest. This is a felony, two felony counts. It was terrible, I know, absolutely. It was awful. And at the time I thought that was the worst thing I could ever possibly experience. Reporter: Dylan's cold reaction after the arrest scared and shocked her. He acted as if he had done nothing wrong. She says she gave him one of her lectures, trying to reason with him. I even talked about the ten command commandments, it's wrong to steal, in no circumstances this is right. And then we responded as most parents would. We took away privileges. Reporter: One night she's frustrated, he's not doing chores, he's withdrawn, she pushes him against the refrigerator. I said, you've got to stop thinking of yourself, you've got to stop being so selfish. But -- excuse me. But I gave him the old mom lecture. And then I said, by the way, today's mothers day and you forgot it. And I don't remember how that confrontation ended. I just remember he softly said, mom, please don't push me, I don't know how much I can control myself. It wasn't a scary thing. It was just him being nice to say, back off, please. Reporter: She says she blamed herself for pushing him too far. Then he went out and he got me a gift. A little -- it was a little water king with african violets in it. And I thought everything was fine. Because he was so -- he was so sweet. When we come back, the secret journals. What do they reveal about the minds of two teenagers responsible for columbine? And what forced sue klebold to finally face the truth about her son? Tonight, what experts tell us might prevent a tragedy.
This transcript has been automatically generated and may not be 100% accurate.