There was nothing particularly original about the scenario.

A gunman walked into a classroom, took students hostage, and then opened fire.

Of the roughly 30,000 fatal shootings in this country every year, a statistically minute number take place this way. But the horror is not diminished by its rarity. And when it happens three times in a week, the inevitable question arises: Was each successive killing inspired by the one before?

"Unfortunately," said James Alan Fox, "the contagion effect can surface very quickly. And the bad news is, things could get worse. "

Fox, professor of criminal justice at Northeastern University, said yesterday that a similar wave of copycat shootings occurred after two students from Columbine High School in Colorado killed 12 of their classmates and a teacher in 1999.

"Ninety-nine-point-nine percent of schoolchildren identified with the victims," Fox said. "But a small percentage identified with the shooters because, not only did they get even with bullies and nasty teachers, but they got famous for it. "

A study of subsequent shootings in the months after Columbine found that "all involved white kids in small towns," Fox said. "The copycat effect would be most pronounced when there is a similarity between the perpetrator and the ones they are idolizing and modeling. "

Whatever the short-term effects have been in setting off a cascade of similarly staged acts of violence, several experts suggested that the national focus belongs on the larger context. That is, the need to reach troubled people, provide mental health services, and address the normalizing of violence in American society.

Fox, the author of The Will to Kill: Explaining Senseless Murder and Extreme Killing: Understanding Serial and Mass Murder, said schools carried a symbolic power as targets.

"If you want to find young kids and get even with society - a school is an ideal place for doing that," he said. "They represent a place where people may have felt unhappy, their self-esteem was threatened, where they were bullied, and where they decide to get revenge. "

The majority of attackers in school settings are motivated by revenge, according to the 2002 "Safe School Initiative," which examined targeted school violence between 1974 and 2000. The 50-page report, by the Secret Service and the U.S. Department of Education, found that 93 percent planned the assault in advance and displayed behavior signaling that they were preparing to harm their targets.

"The copycat theory must be considered since these attacks happened in such close proximity to each other," said Deborah Prothrow-Smith, assistant dean at the Harvard School of Public Health and author of Murder Is No Accident. "But the copycat theory tends to minimize the true cultural aspects of this. "

Prothrow-Smith said, "You've got a socially toxic environment that glamorizes guns and violence. "

Video games, television, films and news constantly project images of people "justifying their wrongs or emotions with violence," she said. "You mix guns in a culture where people are not good at handling difficult emotions like anger, fear, guilt and grief . . . and you have a toxic social environment. "

Having said this, Prothrow-Smith added there was nothing inevitable about the psychological chemistry leading to attacks like this.

"We have a sick man with a gun in a society that justifies violence," she said. "Something was going to happen. What he decided to do might have been influenced by the copycat phenomenon, but normal, healthy people in normal, healthy situations don't watch this on TV and go do it the next day. "

Contact staff writer Melissa Dribben at 215-854-2590 or mdribben@phillynews.com.

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See the federal Safe Schools report via http://go.philly.com/safeschools03