Five years ago today, Eric Harris and Dylan Klebold murdered their classmates and teachers at Columbine High School. Most Americans have reached one of two wrong conclusions about why they did it. The first conclusion is that the pair of supposed “Trench Coat Mafia outcasts” were taking revenge against the bullies who had made school miserable for them. The second conclusion is that the massacre was inexplicable: We can never understand what drove them to such horrific violence.

But the FBI and its team of psychiatrists and psychologists have reached an entirely different conclusion. They believe they know why Harris and Klebold killed, and their explanation is both more reassuring and more troubling than our misguided conclusions. Three months after the massacre, the FBI convened a summit in Leesburg, Virginia, that included world-renowned mental health experts, including Michigan State University psychiatrist Dr. Frank Ochberg, as well as Supervisory Special Agent Dwayne Fuselier, the FBI’s lead Columbine investigator and a clinical psychologist. Fuselier and Ochberg share their conclusions publicly here for the first time.

The first steps to understanding Columbine, they say, are to forget the popular narrative about the jocks, Goths, and Trenchcoat Mafia—click here to read more about Columbine’s myths—and to abandon the core idea that Columbine was simply a school shooting. We can’t understand why they did it until we understand what they were doing.

School shooters tend to act impulsively and attack the targets of their rage: students and faculty. But Harris and Klebold planned for a year and dreamed much bigger. The school served as means to a grander end, to terrorize the entire nation by attacking a symbol of American life. Their slaughter was aimed at students and teachers, but it was not motivated by resentment of them in particular. Students and teachers were just convenient quarry, what Timothy McVeigh described as “collateral damage.”

The killers, in fact, laughed at petty school shooters. They bragged about dwarfing the carnage of the Oklahoma City bombing and originally scheduled their bloody performance for its anniversary. Klebold boasted on video about inflicting “the most deaths in U.S. history.” Columbine was intended not primarily as a shooting at all, but as a bombing on a massive scale. If they hadn’t been so bad at wiring the timers, the propane bombs they set in the cafeteria would have wiped out 600 people. After those bombs went off, they planned to gun down fleeing survivors. An explosive third act would follow, when their cars, packed with still more bombs, would rip through still more crowds, presumably of survivors, rescue workers, and reporters. The climax would be captured on live television. It wasn’t just “fame” they were after—Agent Fuselier bristles at that trivializing term—they were gunning for devastating infamy on the historical scale of an Attila the Hun. Their vision was to create a nightmare so devastating and apocalyptic that the entire world would shudder at their power.

Harris and Klebold would have been dismayed that Columbine was dubbed the “worst school shooting in American history.” They set their sights on eclipsing the world’s greatest mass murderers, but the media never saw past the choice of venue. The school setting drove analysis in precisely the wrong direction.

Fuselier and Ochberg say that if you want to understand “the killers,” quit asking what drove them. Eric Harris and Dylan Klebold were radically different individuals, with vastly different motives and opposite mental conditions. Klebold is easier to comprehend, a more familiar type. He was hotheaded, but depressive and suicidal. He blamed himself for his problems.

Harris is the challenge. He was sweet-faced and well-spoken. Adults, and even some other kids, described him as “nice.” But Harris was cold, calculating, and homicidal. “Klebold was hurting inside while Harris wanted to hurt people,” Fuselier says. Harris was not merely a troubled kid, the psychiatrists say, he was a psychopath.

In popular usage, almost any crazy killer is a “psychopath.” But in psychiatry, it’s a very specific mental condition that rarely involves killing, or even psychosis. “Psychopaths are not disoriented or out of touch with reality, nor do they experience the delusions, hallucinations, or intense subjective distress that characterize most other mental disorders,” writes Dr. Robert Hare, in Without Conscience, the seminal book on the condition. (Hare is also one of the psychologists consulted by the FBI about Columbine and by Slate for this story.*) “Unlike psychotic individuals, psychopaths are rational and aware of what they are doing and why. Their behavior is the result of choice, freely exercised.” Diagnosing Harris as a psychopath represents neither a legal defense, nor a moral excuse. But it illuminates a great deal about the thought process that drove him to mass murder.

Diagnosing him as a psychopath was not a simple matter. Harris opened his private journal with the sentence, “I hate the f—ing world.” And when the media studied Harris, they focused on his hatred—hatred that supposedly led him to revenge. It’s easy to get lost in the hate, which screamed out relentlessly from Harris’ Web site:

“YOU KNOW WHAT I HATE!!!? Cuuuuuuuuhntryyyyyyyyyy music!!! …“YOU KNOW WHAT I HATE!!!? People who say that wrestling is real!! …“YOU KNOW WHAT I HATE!!!? People who use the same word over and over again! … Read a f—in book or two, increase your vo-cab-u-lary f*ck*ng idiots.”“YOU KNOW WHAT I HATE!!!? STUPID PEOPLE!!! Why must so many people be so stupid!!? … YOU KNOW WHAT I HATE!!!? When people mispronounce words! and they dont even know it to, like acrosT, or eXspreso, pacific (specific), or 2 pAck. learn to speak correctly you morons.YOU KNOW WHAT I HATE!!!? STAR WARS FANS!!! GET A FaaaaaaRIGIN LIFE YOU BORING GEEEEEKS!

It rages on for page after page and is repeated in his journal and in the videos he and Klebold made. But Fuselier recognized a far more revealing emotion bursting through, both fueling and overshadowing the hate. What the boy was really expressing was contempt.

He is disgusted with the morons around him. These are not the rantings of an angry young man, picked on by jocks until he’s not going to take it anymore. These are the rantings of someone with a messianic-grade superiority complex, out to punish the entire human race for its appalling inferiority. It may look like hate, but “It’s more about demeaning other people,” says Hare.

A second confirmation of the diagnosis was Harris’ perpetual deceitfulness. “I lie a lot,” Eric wrote to his journal. “Almost constantly, and to everybody, just to keep my own ass out of the water. Let’s see, what are some of the big lies I told? Yeah I stopped smoking. For doing it, not for getting caught. No I haven’t been making more bombs. No I wouldn’t do that. And countless other ones.”

Harris claimed to lie to protect himself, but that appears to be something of a lie as well. He lied for pleasure, Fuselier says. “Duping delight”—psychologist Paul Ekman’s term—represents a key characteristic of the psychopathic profile.

Harris married his deceitfulness with a total lack of remorse or empathy—another distinctive quality of the psychopath. Fuselier was finally convinced of his diagnosis when he read Harris’ response to being punished after being caught breaking into a van. Klebold and Harris had avoided prosecution for the robbery by participating in a “diversion program” that involved counseling and community service. Both killers feigned regret to obtain an early release, but Harris had relished the opportunity to perform. He wrote an ingratiating letter to his victim offering empathy, rather than just apologies. Fuselier remembers that it was packed with statements like Jeez, I understand now how you feel and I understand what this did to you.

“But he wrote that strictly for effect,” Fuselier said. “That was complete manipulation. At almost the exact same time, he wrote down his real feelings in his journal: ’Isn’t America supposed to be the land of the free? How come, if I’m free, I can’t deprive a stupid f—ing dumbshit from his possessions if he leaves them sitting in the front seat of his f—ing van out in plain sight and in the middle of f—ing nowhere on a Frif—ingday night. NATURAL SELECTION. F—er should be shot.’ “

Harris’ pattern of grandiosity, glibness, contempt, lack of empathy, and superiority read like the bullet points on Hare’s Psychopathy Checklist and convinced Fuselier and the other leading psychiatrists close to the case that Harris was a psychopath.

It begins to explain Harris’ unbelievably callous behavior: his ability to shoot his classmates, then stop to taunt them while they writhed in pain, then finish them off. Because psychopaths are guided by such a different thought process than non-psychopathic humans, we tend to find their behavior inexplicable. But they’re actually much easier to predict than the rest of us once you understand them. Psychopaths follow much stricter behavior patterns than the rest of us because they are unfettered by conscience, living solely for their own aggrandizement. (The difference is so striking that Fuselier trains hostage negotiators to identify psychopaths during a standoff, and immediately reverse tactics if they think they’re facing one. It’s like flipping a switch between two alternate brain-mechanisms.)

None of his victims means anything to the psychopath. He recognizes other people only as means to obtain what he desires. Not only does he feel no guilt for destroying their lives, he doesn’t grasp what they feel. The truly hard-core psychopath doesn’t quite comprehend emotions like love or hate or fear, because he has never experienced them directly.

“Because of their inability to appreciate the feelings of others, some psychopaths are capable of behavior that normal people find not only horrific but baffling,” Hare writes. “For example, they can torture and mutilate their victims with about the same sense of concern that we feel when we carve a turkey for Thanksgiving dinner.”

The diagnosis transformed their understanding of the partnership. Despite earlier reports about Harris and Klebold being equal partners, the psychiatrists now believe firmly that Harris was the mastermind and driving force. The partnership did enable Harris to stray from typical psychopathic behavior in one way. He restrained himself. Usually psychopathic killers crave the stimulation of violence. That is why they are often serial killers—murdering regularly to feed their addiction. But Harris managed to stay (mostly) out of trouble for the year that he and Klebold planned the attack. Ochberg theorizes that the two killers complemented each other. Cool, calculating Harris calmed down Klebold when he got hot-tempered. At the same time, Klebold’s fits of rage served as the stimulation Harris needed.

The psychiatrists can’t help speculating what might have happened if Columbine had never happened. Klebold, they agree, would never have pulled off Columbine without Harris. He might have gotten caught for some petty crime, gotten help in the process, and conceivably could have gone on to live a normal life.

Their view of Harris is more reassuring, in a certain way. Harris was not a wayward boy who could have been rescued. Harris, they believe, was irretrievable. He was a brilliant killer without a conscience, searching for the most diabolical scheme imaginable. If he had lived to adulthood and developed his murderous skills for many more years, there is no telling what he could have done. His death at Columbine may have stopped him from doing something even worse.