director of the Brudnick Center on Violence and Conflict at Northeastern University in Boston. "The problem is that those warning signs apply to far too many people. There are a lot of false positives."

Professor Levin suggests casting a broad net in terms of offering help and counseling services to such students. Others note that it's important to educate teachers, students, and administrators about the sorts of behavior they should look out for, as well as what to do when they see them.

In Cho's case, some classmates did in fact convey their concerns to administrators and police. Lucinda Roy, an English professor who taught Cho's poetry workshop, was so disturbed by his writing that she alerted campus police, counseling services, and other officials, The New York Times reported Wednesday.While an FBI report on threat assessments for school shooters cautioned that there's no single profile of a shooter, it included many signs for fellow students and teachers to be alert to, including shared threats or violent fantasies, lack of empathy, low tolerance for frustration, and extreme alienation.

Even before Cho was identified as the shooter at Virginia Tech, many experts and psychologists detailed a general profile that he came to fit almost perfectly. Among other characteristics, mass school shooters tend to be men with psychological wounds from an abusive or troubled past. They are loners whose final outburst is often triggered by a rejection, real or perceived.

Investigators had trouble finding information about Cho because of his extreme social alienation. In both of his plays published online, adult male