Almost anyone who has spent time in a school or workplace will tell you there is always that someone who seems a little off. The guy most likely to "go postal," people tend to say in an off-handed way.
The more the nation learns about the Virginia Tech killer, the more we realize the signs were everywhere. It's not always that easy. So how do you know, how can you protect yourself?
Unfortunately, psychiatrists say there is no way to definitively predict the kind of horrific behavior seen at Virginia Tech.
"There are millions of people who are depressed who are not violent, and there are millions of people who are lonely or write crazy kinds of things who are not violent either," said Dr. Peter Marzuk, associate professor of psychiatry at New York-Presbyterian/Weill Cornell and a published expert on the topic of murder-suicide.
"Even then, most people who are violent do not commit mass murder."
Still, there may be certain indicators that can help predict when an individual is contemplating violence -- as well as ways to protect yourself if the worst occurs.
Past history of violence
"The most important predictor of future violent behavior is past violent behavior," Marzuk said. "If someone is known to have a past of violence, that alone is important."
A past history of violence could include being the victim of abuse, or a history of abusing others.
Display of violence in writing or art
Cho's disturbing writings could be considered a red flag, indicating his deep rage and capacity for violence.
"A person's art is almost like a Rorschach's test," said Dr. Igor Galynker, director of the Bipolar Family Treatment Center at Beth Israel Medical Center in New York, N.Y. "Cho's plays are bizarre beyond odd. They are a product of a psychotic mind."
Talking about violence or suicide
Individuals who seem to be "simmering" -- or even making overt threats of violence -- may also be more likely to commit violent acts.
"Once you start making threats, that moves it up into another category," Marzuk said.
Verbal expressions of violent tendencies are also among the most ignored warning signs, Galynker said.
"A person can broadcast his intent to kill loudly, publicly and repeatedly, and still be ignored because mass murders are very rare and people close to the future murderer have never seen anything like that before," he said. "Obviously, only hindsight is 20:20."
Loneliness and social isolation
From student accounts it seems clear that Cho was profoundly isolated, which suggests that few may have been able to pick up on his violent tendencies before they became dramatically apparent.
"It's becoming clearer that Cho was an extreme example of a loner who was unable to communicate with others about things that happened that were distressing to him," said Dr. Redford Williams, director of Duke University's Behavioral Medicine Research Center in Durham, N.C.
Stalking and other antisocial or criminal behavior
Cho's roommates note that prior to his rampage, he had stalked some of the female students on campus using the Internet. Dr. Kathyrn Moss, attending psychiatrist in the Personalities Disorders Clinic at NewYork-Presbyterian/Weill Cornell, said this alone could have been a chance to intervene.
"I think it's very important that when people harass you or treat you in illegal ways that you understand that you should take legal action," Moss said.