Psychologist Peter Langman, one of the country's foremost school shootings experts, at his office in Allentown, Pa. (Ricky Carioti/The Washington Post)

When Sue Klebold began researching a book last year about her struggle to comprehend her son’s rampage at Columbine High School, she turned to a researcher who has been grappling with the shooting for nearly as long as she has.

Peter Langman, a 56-year-old psychologist in private practice, has studied school shootings for more than a decade, becoming one of the country’s premier experts on the phenomenon. He has spent so much time reading the journals of Dylan Klebold and his rampage partner Eric Harris — they killed 13 people and wounded dozens more on April 20, 1999 — that he sometimes dreams about them.

Langman had heard that Sue Klebold was writing a book. Still, it astonished him when she reached out to him. “The mother who raised her son from birth and saw him every day was seeking insight from someone who had never met him before,” he said.

[Does ‘damaged masculinity’ explain Columbine and other mass shootings?]

The clues to Columbine — and so many other school shootings — are in his office closet.

Sue Klebold, mother of Columbine shooter Dylan Klebold, turned to Peter Langman as she was writing her book about her son, "A Mother's Reckoning: Living In the Aftermath of Tragedy.” (Jesse Dittmar/FTWP)

Out of patients’ view, in thick binders stacked one atop another, Langman stores police reports, court transcripts and the twisted writings of hundreds of school shooters. Langman has posted more than 57,000 pages of documents on his website, analyzing the material in two books, scholarly papers and many case histories.

Langman sees his research on school shootings “as a moral obligation.” He often lies in bed at night highlighting police reports. His wife and daughter proofread his papers. His son helped set up his website, which Langman has spent more than $20,000 maintaining. There are 133 shooters in his online database, which grows every year.

From the 1960s to the 1980s, there were three to eight multi-victim incidents per decade. Now there are about 20. And Langman has just finished a paper, yet to be published, showing that the incidents are morphing: The shooters are older, they are no longer almost exclusively white, and the attacks are more deadly.

Langman trains educators, mental-health professionals and law enforcement groups across the country, yet he’s frustrated that so much emphasis is placed on lockdowns and fortified doors instead of on widespread education about prevention — something the Centers for Disease Control and Prevention could do were it not for a controversial funding ban on gun-violence research.

Mark Rosenberg, former director of the CDC’s National Center for Injury Prevention and Control, said it is a “huge mistake” that research into school shootings isn’t done at the federal level, especially given that some researchers now think that, as a result of intense media coverage, the incidents are contagious.

“There are people at the CDC who very much want to do this work,” Rosenberg said, “but they can’t.”

Langman said he thinks that educators, parents and even students should be taught to look for early warning signs, such as oblique hints in school assignments, during hallway chatter and on social media. Too often, parents find themselves putting the puzzle together after the bloodshed.

Two survivors from the 2007 Virginia Tech shooting share their story of coping in the aftermath of a tragedy and how it still impacts them to this day. (Alice Li/The Washington Post)

That’s what happened to Sue Klebold. And she needed Langman to help her.

“I had to understand how my son could do such a thing,” she said in an interview. “I had to make some kind of sense of it so I could live with myself.”

Langman said the closest he has been to the “firsthand suffering in one of these incidents” were two long phone calls with Sue Klebold.

She began their first conversation with a question: “What happened to Dylan?”

A string of potential shooters

On the day of the Columbine shooting 17 years ago, Langman was a doctoral intern at a children’s psychiatric hospital.

“Like the rest of the nation, I was shocked and saddened by the massacre, and I knew the attack constituted a major event in the country,” Langman wrote in “Why Kids Kill: Inside the Minds of School Shooters,” published in 2010. “I did not anticipate, however, the impact the attack would have on my life.”

Ten days later, as the nation was gripped by the fear of copycat attacks, a 16-year-old boy was taken to Langman’s hospital. He had written a series of disturbing essays about sex, murder and dismemberment. He had a hit list on his website. School officials thought he might “go Columbine.”

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Langman was asked to determine whether the boy was actually a threat.

“I had nothing, really, to go on,” Langman recalled. “There was no literature on this.”

Langman immediately began asking himself questions that would eventually underpin his theory for how school shooters should be classified. Was this teenager psychotic? Was he a psychopath? Was there something else going on? Langman wasn’t sure, but he knew the child was dangerous, so he suggested extensive treatment.

A couple of weeks later, in came another potential school shooter. Then another. And another. Langman became the go-to guy for the evaluations.

“Once you’ve done them,” he said, “you’re the guy who has done them.” Langman started compiling charts and graphs to find patterns among the potential threats he was now seeing regularly, filling in the research gaps himself.

“As a psychologist, I felt an ethical obligation to know as much as possible about the subject,” he later wrote. “Although I recognized these gaps, I was not sure I wanted to be the one to fill them.”

Langman entered private practice in Allentown, no longer seeing potential school shooters frequently. He spent his free time writing plays and poetry.

Meanwhile, the shootings continued. In 2003, John McLaughlin, 15, killed two students at a high school in Cold Spring, Minn. In 2005, 16-year-old Jeffrey Weise killed his grandfather and another person, then seven more people at his high school on an Indian reservation in Red Lake, Minn.

And then, on Sept. 13, 2006, Kimveer Gill, a 25-year-old college dropout, went to Dawson College in Montreal and shot 20 people, one fatally. Gill’s friends told reporters that he was fascinated by Columbine.

“That was the day I decided I was going to really do something about this,” Langman said. “It was a problem that just wasn’t going away.”

He collected documents online from previous school shootings, including more than 20,000 pages of material from Columbine. He launched his website, posting materials as he tracked them down. And he read, underlined and catalogued nearly every shred of paper he got his hands on.

Three years later, he published the first of his two books, in which he uses the documents collected in his closet forensic archive to lay out what he thinks are three categories of shooters at secondary schools and college campuses: traumatized, psychopathic and psychotic.

[Most mass shooters aren’t mentally ill]

Traumatized shooters are those who weren’t “simply abused children but experienced multiple difficulties that caused unstable, overwhelmingly stressful lives.”

Eric Houston, a 20-year-old who in 1992 shot 14 people, killing four, at Lindhurst High School in California, is one example. He grew up in a family “plagued by incest, alcoholism, physical abuse, suicide and murder,” Langman notes. Houston’s first victim was a teacher he said molested him.

“If I die today,” he wrote in a note before the attack, “please bury me somewhere beautiful.”

Psychopathic shooters are those, like Eric Harris, who lack any semblance of empathy and are totally callous in their regard for anyone’s well-being but their own. “It has been confirmed, after getting my yearbook,” Harris wrote in one of his rambling online posts, “that the human race isn’t worth fighting for, only worth killing.”

‘Confused, despondent’

And then there are psychotic ones, like Dylan Klebold. Of all the shooters Langman has researched, Dylan Klebold is, to him, the saddest and most fascinating case. He is the only school shooter to get his own chapter in any of Langman’s books.

The teenager went to prom with a pretty girl a few days before the attack. He had just visited his future dorm room at college. He liked to bowl. His friends — contrary to the loner stereotype, he had a social life — thought he was shy, maybe a little goofy.

But in his journals, Dylan Klebold was an entirely different person — “lost in fantasies, confused, lonely, despondent, self-deprecating and yet elevating himself to godhood,” Langman writes. Speaking to Dylan Klebold’s mother, Langman explained his theory that her son had a schizotypal personality disorder.

“They withdraw into a world where reality and fantasy are not always distinguishable,” Sue Klebold said in her book. “These are not full-on delusions, but a fuzziness in the boundary between what is real and what is not.” Langman told her that her son “created a fantasy where he was a godlike being.”

He remade himself in Harris’s image. “He was willing to do anything — even kill people — to win Harris’s approval,” Langman wrote in his second book, “School Shooters.” That, Langman thinks, is what happened to Dylan Klebold.

In some ways, Langman’s conversations with Sue Klebold were a form of therapy for her.

“I felt very drawn to him,” Klebold said. “He spoke with such compassion for Dylan. He conveyed to me that Dylan was a human being and he was not an evil monster.”

Their first conversation ended with Sue Klebold sobbing. In his trove of documents, Langman told her that he found four instances during the attack when her son let potential victims get away. This happened only when Eric Harris wasn’t close by, a telling detail.

“When something happens like this, you want to find evidence to prove to yourself that it was less terrible than you thought,” Klebold said. “That was a huge gift to me. Even though what Dylan did was horrible, it helped me to hear that.”

In an email, Langman assured her: “Nothing you did or didn’t do caused Dylan to do what he did. By the end of his life, Dylan’s psychological functioning had deteriorated to the point that he was not in his right mind.”

For Langman, the experience was overwhelming. “I was choked up with emotion,” he wrote in a blog post after her book was published. “To come into contact, even briefly, with Sue’s suffering was a profound experience.”

There have been times over the years when he has had to step away from the work.

“It infiltrates the mind,” he said.

But moments like the one with Sue Klebold, when he could offer some clarity to the incomprehensible, get him back at his desk analyzing documents. He’d like to stop having disturbing dreams about Columbine. He’d like the shootings to stop.

He knows they won’t.