This is the second in a series on Columbine, 10 Years Later
Two years ago today, Seung-Hui Cho slaughtered 32 students at Virginia Tech, claiming to have been inspired by the two teenagers who carried out the Columbine shootings, calling them "martyrs" in delusional diatribe he videotaped for the world.
"You had a hundred billion ways to have avoided today," he said on video aired on national television. "But you decided to spill my blood. You forced me into a corner and gave me only one option. Now you have blood on your hands that will never wash off."
In 1999, when Eric Harris seduced his friend Dylan Klebold to open fire at Colorado's Columbine High School, killing 13 and injuring 24, no one had a definitive profile of the school shooter.
Today -- as the 10th anniversary of the Columbine tragedy on April 20 approaches-- experts say they can't predict which teens will go on a suicide-driven rampage..
"Not all psychotics or psychopaths are going to kill and most are not dangerous," said veteran FBI behavioral scientist Kenneth V. Lanning.
In 2000, The National Institute of Justice joined forces with the Secret Service and the Department of Education to assess ways to prevent school shootings.
Looking at 37 school shootings to find patterns in school-aged assassins, the study concluded that all are male and most are loners, with some kind of grievance. More than half had revenge as a motive.
"But that's typical of almost every adolescent," Lanning told ABCNews.com.
"The biggest problem with school shooters is the false positives and false negatives," he said. "How many people in any school have all these characteristics and will never shoot anybody."
Reports from the Department of Education show schools to be largely safe. But high-profile shootings have caused anxiety among parents, students and their teachers.
Contrary to public perception, school shootings declined after 1993, although there were copycat incidents from 1997 to 1999 "stimulated" by unprecedented media coverage, according to the National School Safety Center.
Still, they continue to capture the nation's imagination with images of vengeful outcasts, trench coats and bullied loners.
Some of the conclusions of the federal report were borne out in the Virginia Tech tragedy: shooters tend not to snap, but usually plan months or years in advance and often tell a friend or classmate.
Cho reportedly began planning his attack more than a month before the 2007 massacre, when he purchased his first gun. His video, made in combat gear, appears to have been made at least six days before the attack.
Harris and Klebold also planned in advance, with journals and "basement tapes" chronicling their plan to blow up Columbine High School.
But the comparisons end there.
And when the public throws around words interchangeably -- like psychotic and psychopath -- they underscore the need for better mental health education.
"When we see a person go off the deep end in a shooting, we look in hindsight and piece it together," Lanning said. "Frequently all the warning signs were there and we should have known. But you get warning signs one and two from the mother, three and four from the teacher, five and six from the counselor and probation officer."
Schools need to find better ways to accumulate information and share, within the boundaries of privacy.
"I can't just pick up any Tom, Dick or Harry under the sun," Lanning said. "I'd get sued. The bottom line is this is what American society struggles with all the time, balancing public safety with freedom and rights."
The most iconic of all school shooters -- Harris and Klebold at Columbine and Cho at Virginia Tech -- could not have been more different, according to most experts.
Cho, 23, was mentally ill and delusional -- a psychotic, mental health experts have said. As early as 2005, two female students at Virginia Tech complained about the anxious son of Korean immigrants, and a state court declared him to be at risk for suicide, referring him for psychiatric treatment.
Like the Columbine killers, Cho took his own life in the rampage, but mental health experts have said he may have suffered from bipolar depression or schizophrenia
Unlike Cho, Harris was a psychopath -- controlling, manipulative and sadistic, according to journalists, psychologists and law enforcement experts who studied the case. Psychopaths are in touch with reality and rational, and nearly always well-liked and charming, according to experts.
Klebold was a lonely depressive, full of mood swings and suppressed emotional rage, according to psychiatrists involved in the case.
But together, the Columbine pair was a "deadly dyad," according to Dave Cullen, a journalist who has covered the tragedy a decade and published a book, "Columbine," this month to coincide with the 10th anniversary.
Neither the Chos, nor the Columbine families ever talked freely with the press about their sons' actions. But in 2004, on the fifth anniversary of the tragedy, Tom and Sue Klebold, who still live in Littleton, Colo., responded to an article in the New York Times.
The Klebolds told reporter David Brooks that they objected to the way their son had been described as "depressive" and blamed the toxic atmosphere of teasing at the high school.
But Cullen said that unlike Cho, who was not well-liked and kept to himself, Harris and Klebold had an active social life and were bullies, rather than bullied.
"We always get the wrong answer because we phrase the question wrong," Cullen said.
"Everyone says, 'Why did they do it?' That gets you in trouble. There isn't one thing to explain Columbine," he said. "Why Eric did it and why Dylan did it -- they are polar opposites. You can't fuse it into one.
"It's the same thing with school shooters," he said. "We still go the same route and look for a profile and think we've got one -- outcast, loners and bullies. In two-thirds of cases, they don't apply. There are three or four or five profiles."
According to former FBI psychiatrist Frank Ochberg, who worked in hostage negotiations in the 1970s, Cullen's book "hit the nail on the head."
"The general public has its own idea about evil and how it gets created, distilled and powered," Ochberg told ABCNews.com. "We have so many archtypes."
Harris was a "budding psychopath, a person without a conscience," he said. "He got his satisfaction by dominating."
"Psychopaths don't feel guilty because they are blind to guilt," Ochberg said. Harris also had sadistic tendencies, which propelled him to "seek vengeance."
Klebold, on the other hand, was depressed, with pent up anger and "mood regulation problems," but together, they had "violent creativity," Ochberg said.
"Eric needed Dylan's emotionality and impulsiveness, and Dylan needed Eric's cold psychopathy," according to Ochberg.
While Klebold longed to end his life, as seen in his journals, for Harris, suicide was not a concern, according to Ochberg.
"His life wasn't as important as his appetite," he said. "He turned a comic book fantasy into reality. The purpose was not to kill himself, but it was an option, He needed power."
According to FBI trainer Lanning, psychopathy and psychosis can overlap, but the public wrongly uses the terms interchangeably.
Psychotics are mentally ill, delusional and out of touch with reality; psychopaths can be "wheeler-dealers and manipulators," he said.
Most psychotics are not violent, but their nature is unpredictable, he said.
"Neither is necessarily a killer," said Lanning. "But society tends to focus on those common violent crimes."
Whether psychopaths -- sometimes called sociopaths -- lack a moral compass is up for debate, according to Lanning.
"They have a conscience," he said. "It's just that it's their own, not society's.
"A sex offender may kidnap and rape and mutilate women, but if you put him in prison next to the guy who fondles children, he thinks he's a sick pervert," he said.
Some sub-cultures admire the character of psychopaths.
"If you're a con artist and cheating people out of their savings, the best character to be is a psychopath," Lanning said.
When raised in a nurturing family, they tend to be thrill-seekers -- race car drivers and mountain climbers, "which is more acceptable," he said.
In fiction, they are self-focused characters like J.R. Ewing from television's "Dallas" and Scarlett O'Hara from "Gone With the Wind."
Still, one of the lessons of Columbine and Virginia Tech is understanding the complexity of the human psyche and the difficulty of identifying which teens will cross the line and become a killers.
"Remember Charles Atlas?" asked Lanning, who cites a comic book ad that featured a 90-pound weakling who gets sand kicked in his face, builds his body up and seeks revenge.
"The idea of avenging through physical force for slights against is the age-old dream of adolescent boys," he said. "You are an outcast, you get picked on and you want to get even."
ABC's information specialist Nicholas Tucker contributed to this report.