Transcript for How Columbine Killers Dylan Klebold, Eric Harris Were Different: Part 2
Look at a kind of class picture. Can you pick out which ones will grow up to be school shooters? Almost two-thirds will be from two-parent homes. Nearly half of the kids will do well in school. 73% of them will never have been arrested before. And yet every child you see here will become a school shooter. In total murdering 67 people. The child in this picture is Dylan klebold. I don't think he was always a violent young man. I think it evolved over time. I think it took a lot of time. Reporter: We brought in former FBI agent Mary Ellen o'toole, one of the world's leading profilers of the criminal brain. She has analyzed thousands of pages of evidence on columbine and Dylan klebold. You don't think school shooters just snap one day? Absolutely not. I have not ever seen where that has happened. I think he was in a very destructive friendship which was very powerful. Maybe even more powerful than what we think. Reporter: There is no way to continue telling this story without going back to another little face in that class picture. A friend who was with him breaking into the van. Eric Harris. He would become the other shooter at columbine that day. He has also begun to keep a secret journal, and his journal is filled with venomous threats, graphic fantasies about revenge on people who have insulted him. People engulfed in flames, detap dated heads. According to o'toole it signals a personality disorder, a psychopathic brain. These are people without a conscience. These are became without empathy, without guilt. Reporter: Is Eric Harris really different from Dylan klebold? Another expert on columbine, Dr. Peter Lang man, insists he is. Eric liked to draw weapons, swastikas, wrote about the nazis. Dylan drew hearts. Dylan wrote about his search for true love. Eric when he does refer to girls, his fantasy is raping them. Reporter: In their senior year, eight months before columbine, the two boys spent more time together. They liked violent movies and making little movies of their own. Trying on what it's like to be tough guys. Eric is smooth. I think we might have to get some more weaponry. Reporter: Dylan stumbles. It's a good thing we uh -- we ordered those radio -- Reporter: The camera shuts down. Eric writes, everyone is always making fun of me because of how I look, how weak I am. Guns, I need guns. Dylans and for one too. And I had told him no. Reporter: Sue klebold said she used to look through his room during his junior year, but by his senior year she decided to respect his privacy with distance and regret she now says how wrong that decision was. Would you ransack his room now? I would. I'd do it as if his very life was depending on it. And I would do it with love. Reporter: The boys were already getting their hands on guns and hiding them. A girl, a school friend old enough to buy them legally, got them three at a gun show. She trained at a range. And Dylan klebold writes, he now has a choice. Committing suicide or going nbk with Eric. Nbk, natural-born killers, the violent revenge movie by oliver stone. Which brings us to the central question about Dylan klebold. Why did he go from suicidal to homicidal? Did Eric infect him with a kind of pathological virus? Or was it Dylan's presence that reinforced Eric's violent fantasies? Do you think Dylan klebold knew right from wrong? Yes, absolutely he did. But it did not preclude him from being able to participate in the planning and to carry it out. Do you believe in evil? I don't think so. I don't think I do. Reporter: But o'toole says there were signs that were missed and that it's striking how many other people might have raised an alarm. It turns out the boss at the pizzeria where Dylan worked had seen the two boys experimenting with a pipe bomb. And perhaps most shocking of all the police knew Eric had a website. Ten pages of savage threats. Some frightened parents had alerted them. Police started to draw up a search warrant. But thinking they don't have enough do nothing. Sue klebold says for months and months her mind was looking for any way to deny the truth. I believed this was a moment of madness. I believed this was some impulsive fluke that happened suddenly. Reporter: It wasn't until six months after columbine her denial is shattered. The lead investigator in the case who brings them face-to-face with the inescapable truth of the evidence. Dylan took part in months of planning. And there he was on videotape recorded as a kind of countdown to the attack. It was horrible to see those tapes. They were posturing. They were acting tough. They were talking about all the horrible things they were planning to do. Reporter: But as we said, she lives in the hell of that contradiction. She has said, I saw the end product of my life's work. I had created a monster. But still, inside her, a mother's love for a son she lost. She hears the final word on the tape. He says, "Hey, mom, I got to go." Just want to apologize for any crap this may bring. Just know I'm going to a better place. Good-bye. He said mom. Just the fact that he said it meant a lot to me. Yeah. Reporter: The tapes never released. We're told they've now been destroyed. Since columbine, law enforcement says there have been 50 rampage school shootings. ABC news has estimated at least 79 thwarted plots. More than half of them mentioning columbine. So what can we do about the shooting at schools? Prevention can work but how? In dozens of cases it is fellow students who overheard something, tipped off authorities. Most of the time it is nothing. But maybe it's just that one peek into what could be the next columbine. Reporter: Last week we tried to call every single family. Some of them expressed anger. Others said they preferred to move on. Some said they wanted to offer sue klebold their prayers and grace. So many people, as someone said, sentenced to a life of grief with no parole. After our interview is over, she walks outside in the foothills of the rocky mountains and tells us sometimes she finds herself drawn to a place that has a plaque with these words. It brought a nation to its knees. What have we learned? It's the columbine memorial. I feel a kind of unwelcome there. Like, of course. That perhaps I'm intruding. Reporter: But someday if you go to that memorial, you just might see a gray-haired woman sitting there quietly alone. Sometimes I just sit there. And think. And I tell them I'm sorry. Reporter: For "Nightline," I'm Diane sawyer. Our thanks to Diane sawyer. Sue klebold has written a memoir called "A mother's reckoning: Living in the aftermath of tragedy." All the book's profitsless toward research and charitable foundations focusing on mental health issues.
This transcript has been automatically generated and may not be 100% accurate.