The gunman who went on a shooting rampage at Virginia Tech exercised brutality so extreme and so calculated, it's hard to believe.
Shooting suspect Cho Seung-hui allegedly chained emergency exits to prevent students and faculty from escaping, and he reportedly lined up victims systematically before shooting them to maximize the number of dead.
Presumably, the 23-year-old Cho, a Virginia Tech senior and English major, was at one time an average college student. So how could he have snapped?
Since the Columbine High School shootings in 1999, doctors and psychologists have attempted to delve into the dark minds of student killers. They've found that deep-seated rage is often what drives young people to kill.
"This is not the difference between a good day and a bad day for the average ordinary person -- this is someone who is severely troubled almost by definition," explained Catherine Newman, Princeton University professor and author of "Rampage: The Social Roots of School Shootings." "You don't do this unless you are severely troubled."
Newman said it's incredibly difficult to pinpoint someone who is deeply disturbed.
"The Secret Service has studied this, and they are in the business of predicting rare events and they have concluded that there is no way to identify the kind of person who is going to do this in advance," she said.
But former FBI agent Brad Garrett believes that student killers leave clues before they commit their crime.
"These people always leave clues," he said. "Because you don't go from having a job and a girlfriend and a family to mass murder unless you're insane."
Garrett said that most school shooters tend to be mentally stable.
"If you look at the profile of school shooters, they tend not to be crazy. This is something that builds up over years where they get into a place where they start dehumanizing people and they're very inappropriate in their behaviors," he said. "There are a lot of signs that something could potentially occur."
Becoming reclusive or temperamental can be a sign that someone is struggling with rage.
"A person, for example, might withdraw or become increasingly tense, angry, frustrated, quiet," said University of Southern California psychiatry professor Bruce Spring.
Garrett guessed that relationship troubles could have pushed Cho over the edge. Cho was reportedly involved with a woman who went to another college.
"I'll be surprised if this is not a bad domestic situation," he said. "He's probably had failed relationships before. And he basically feels his life has caved completely in and he has no place else to go."
Though nothing can be done to undo the havoc Cho wrecked, Spring believes people can learn from Monday's Virginia Tech massacre to prevent school shootings from happening in the future.
"If we pay attention to each other and listen to each other and really inquire briefly about what's going on with someone in attentive way, we may be able to pick up on something and understand something," he said.