His interest wasn't merely academic. Langman, a child psychologist, had been asked to evaluate a teenager who posted a hit list on his Web site.
"To be sitting face to face with someone who was thought to be a potential risk for doing something like a Columbine attack was very intense," Langman says now. "A lot was riding on what we did with him. This was a potential mass murderer."
Since there was very little research at the time to guide him, Langman says, he felt an "ethical obligation" to learn all he could about the psychology of school shooters. The result of his decade-long inquiry: a book that plumbs the lives of 10 notorious school shooters - including Columbine killers Eric Harris and Dylan Klebold and Virginia Tech gunman Seung-Hui Cho - to draw conclusions about what set them off.
In "Why Kids Kill: Inside the Minds of School Shooters," released by Palgrave Macmillan just before the 10th anniversary of the slayings in Littleton, Colo., Langman writes that most of the shooters were severely mentally ill, their defective personalities and disordered minds causing existential rage that found its expression in mass murder.
Harris, 18, was a psychopath - rage-filled, egotistical, lacking conscience, writes Langman. Klebold, 17, was psychotic, suffering from paranoia, delusions and disorganized thinking. Together, they fatally shot 12 students and a teacher and wounded 23 before committing suicide at Columbine on April 20, 1999.
To diagnose the conditions of Harris and Klebold, Langman examined 27,000 pages of records from the Jefferson County Sheriff's Office, including 5,000 pages previously unseen. While others have pointed out that Harris exhibited the classic behaviors of a psychopath, Klebold was more of a mystery. His journal, released publicly in 2006, gave Langman crucial insights into his personality.
"The biggest eye-opener was the extent to which Dylan Klebold really was mentally disturbed. That was not in the literature, not in the media accounts. To realize that, you had to see his journal," says Langman, clinical director of KidsPeace, a 127-year-old Pennsylvania-based charity with treatment centers from Maine to Florida. "His journal is very fascinating, a very disturbed piece of writing."
Like Klebold, four other psychotic shooters profiled by Langman "were suicidally depressed and full of rage at the inexplicable unfairness of life," writes the 49-year-old psychologist. "In addition, they were not living in reality. They all believed that people or monsters conspired to do them harm. ... They were confused and desperate and lost in the mazes of their minds."
At first, Langman's conclusions might sound obvious: These kids would have to be crazy to go to their school and open fire. But the public and the media, especially in the immediate aftermath of a school shooting, have usually focused on other factors: the killers' fascination with violent movies and video games, their easy access to guns, even the side effects of psychiatric drugs.
Langman says some of these may have been factors but do not by themselves explain rampage attacks in places like Littleton and West Paducah, Ky.; Jonesboro, Ark.; and Springfield, Ore. Millions of kids watch violent movies and live in households that harbor firearms. Yet only a few have ever gone on to become mass murderers.
The most prevalent misconception about school shootings, Langman contends, is that they are perpetrated by loners or outcasts striking out against classmates who bullied them. In reality, most shooters were teased no more or no less than their peers, most had friends, and most of the victims were targeted at random.
But to a public grasping for easy answers, "retaliation makes sense. Kid's picked on, comes to school, gets revenge. There's a clarity to that that we can all relate to," Langman says in an interview in his small, sterile office north of Allentown, its shelves packed with thick diagnostic manuals and books on therapy, psychology and child-rearing.
"I think a lot of people like a simple answer," he continues, "because if there's a simple answer maybe there's a simple solution."
While writing extensively about Columbine, Langman also focuses on the blood-soaked academic year of 1997-98, when attacks at six schools horrified the nation.
He draws on the work of Katherine Newman, a Princeton University sociologist whose 2004 book "Rampage: The Social Roots of School Shootings" focused on West Paducah and Jonesboro, two of the earliest attacks. Newman is an anthropologist, not a psychologist, so she approaches the question of school shootings from a different perspective.
While many shooters undoubtedly suffer from mental illness, Newman says, their attacks can't solely be attributed to their psychiatric conditions. Psychology, for example, can't explain why school shootings happen in small towns and not big cities.
"Shooters are trying to accomplish something. They are trying to change the way that other kids in their peer groups define them," she says.
Langman agrees: Mental illness alone is not the answer, if only because the overwhelming majority of troubled kids are not homicidal. Rather, rampage attacks are caused by "complex combinations of environmental, family, and individual factors" that vary from perpetrator to perpetrator, he writes.
That doesn't mean they can't be predicted - and stopped.
Langman talks about some of the teens he evaluated. One was a 14-year-old who "just stared and stared." Another beat up his mother and calmly contemplated a killing spree. A third built bombs and had a hit list. But not all of them posed an imminent danger.
How did he know? His conclusion depended on whether they had engaged in "attack-related behavior" - that is, taken concrete steps toward an assault.
In the wake of Columbine, such threat assessment has become commonplace at schools around the nation. But Newman and Langman say each generation of kids needs to be taught what to look for, because today's teens view Columbine as ancient history. And because a school shooting is a relatively rare event, it's easy to get complacent.