Ten years after the traumatic killings in Colorado, journalist Dave Cullen’s new book replays the deadly day, its causes and the author’s own obsession.

Dave Cullen intimately understands the emotional rollercoaster onto which he lures readers of his comprehensive history of the Columbine shootings. He knows because he’s been riding it ever since he started covering the attack on April 20, 1999, when Eric Harris and Dylan Klebold opened fire at their suburban Colorado high school, killed 13 and injured 24, and then committed suicide.

Ten years is a long time to work on the same story, especially one so grueling, but the freelance journalist found himself unable to walk away.

“I thought I was done,” Cullen says from his home in Denver. “I was like, ‘No more dead children, ever. I’m going to write about happy things for awhile.’ But things kept sucking me back in. I wanted to know why the killers did it. I got more and more frustrated. ‘Why don’t we know?’ I’d think. The answers we had were ridiculous: A feud with the Trench Coat Mafia or the story that the boys were loners and outcasts. I was always a little skeptical that someone had bombs to kill 500 people because of that. It just didn’t ring true.”

In “Columbine”, Cullen addresses the rumors and outright untruths that surfaced in the wake of the shootings and presents a fascinating portrait of the victims, the killers and a community. Though he was one of the reporters who regularly broke the stories – he was first to report that the repeated rumor that Cassie Bernall had testified to her faith before dying was untrue – he maintains a third-person narrative for clarity’s sake.

Cullen, who was treated for post-traumatic stress syndrome while covering the story, writes compassionately about the families scarred by death or severe injury, and the reactions of the young survivors engaged him deeply. He’s still haunted by the day after the shootings, when so many of them gathered in nearby Clement Park.

“I wasn’t expecting all those kids to have these blank expressions,” Cullen says. “If you’ve been in a theater with a young audience, you see them tugging each other’s hair, laughing, all kinds of horseplay. There’s a teenage nervous energy you can feel. In that park with a thousand kids, there was no energy coming out of them. Just silence. These kids were like walking zombies, and that unnerved me.”

Cullen “didn’t feel right asking them anything,” but when he finally approached a group of boys, he discovered they wanted to talk.

“They said, ‘I have no more tears. I cried til 2 a.m., and it stopped suddenly.’ Or ‘I had to get away from my mom. She was trying to hug me too much.’ They all had similar stories. They knew they were shut down. You know that David Bowie song “Changes” and the line ‘They’re quite aware of what they’re going through’? These kids knew what they were going through but didn’t know how to cure it. … I was impressed by them but worried about them.”

Structuring the book with its numerous story lines – the kids, the killers, the cops, the media, the reaction of local churches – was a headache. Cullen tried to remember that his audience would need a break from the relentlessly grim chapters and balanced them with rare upbeat stories, such as the recovery of Patrick Ireland, who survived the carnage in the library.

Cullen also struggled with the writing. The chapters on Coach Dave Sanders, who was shot and bled to death as S.W.A.T. teams painstakingly swept the school, thinking the gunmen were still alive, were the hardest, he says: “I was crying when I was writing it. I couldn’t see the screen. I was just typing blind. I could only stand it for so many minutes a day.”

A more surprising difficulty arose when he tried to write about Klebold’s funeral. On one hand, Cullen felt that writing from the killers’ perspectives, using their journals and videos, “wasn’t disturbing. It was like looking at disease under a microscope.” He listened to the hardcore music Harris liked and tried to watch Klebold’s favorite movie, David Lynch’s “Lost Highway” – “I couldn’t even finish it” – and otherwise attempted to immerse himself in their dark world. But the funeral scene stymied him.

“I couldn’t understand it, even though I was sympathetic to the parents,” Cullen says. “I went to see my shrink, and she said, ‘You’re feeling sympathy for Dylan.’ I said, ‘No, I’m not!’ I got really mad. The killers were bad. They don’t deserve sympathy. I said, ‘I don’t feel anything for them,’ but I realized I did, and realized I needed to stop treating the killers contemptuously.

“I had a rule for everyone. Each story line had to be from the perspective of the lead character. I tried to reflect the point of view of that person and how they were seeing the world – except the killers. They didn’t get that treatment because they didn’t deserve it. I realized that was a mistake. … I was reading back a passage from Dylan’s part, and this line jumped out at me, a snarky comment by the narrator. I thought, ‘What the hell is “that” doing in there?’ I had allowed myself to be as contemptuous as I wanted to be. I was trying to prove I thought they were bad and evil, and that was getting in the way. My contempt for these kids is not the point. Obviously they did a horrible thing. I don’t need to reinforce that by kicking them. If I’m holding Dylan at arm’s length and being contemptuous, how am I understanding him?”

“Columbine” also takes a hard look at errors by the Jefferson County authorities, namely the fact that an affidavit for a search warrant of Harris’ home was never filed with a judge. Even though we can’t know if an arrest would have stopped Harris – psychopaths don’t get cured, Cullen emphasizes, adding that “he was financing his operation on wages from Blackjack Pizza … Imagine if he’s making 50 or 100 grand. Then he’s Timothy McVeigh” – that affidavit should not have fallen through the cracks.

Police “get zillions of reports from moms who say, ‘I have a juvenile delinquent in my neighborhood,’ and they’re not criminals,” Cullen says. But not following up on the physical evidence of finding an unexploded bomb, combined with specific threats on Harris’ website, is a different story. Harris had “motive, means, opportunity. He’s talking about it specifically, and he is acting. He’s found out how to make the bombs and how to get materials. He’s putting his ideas into action, and that’s the moment that house should have been searched.

“In the post-Columbine world we live in, those things are taken seriously. Now the number of shootings is outnumbered by the number of failed plots … but a lot of schools adopted zero tolerance policies that are ridiculous. We need to take a middle ground that makes sense. React but not overreact. We shouldn’t make high school like the airport where you can’t make a joke about hijacking a plane, or they will arrest you. We can’t make high school a joke-free zone. It’s too long, and they’re too young. We need reasonable middle ground. Kids need to know that if I report my buddy who I think is a good kid, I’m not going to get him expelled if there’s nothing wrong. But if there is, at least a bunch of my friends didn’t die.”