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Monday’s rampage at an Amish school in Pennsylvania, the nation’s third deadly school shooting in less than a week, was another in a series of attacks in which four exurban schools were targeted in the last month by intruders committing murder, mayhem and sexual assaults.

A drifter terrorizing a Colorado school kills a fleeing teen girl, then himself. A deranged ex-student invades a high school in North Carolina, wounding two after first killing his father. A gunman looking for an old girlfriend bursts into a Vermont elementary school and kills a teacher. And then came Monday’s shooting.

Save for a disturbed teenager killing a principal in a rural Wisconsin school Friday, the recent rash of shootings were done by outsiders going against a decade-long trend of insular school violence committed mostly by students.

“It’s a disturbing change,” said William Lassiter, manager for the Center for the Prevention of School Violence in North Carolina. “We’ve never seen anything like this.”

Of the 400-plus deaths in school violence in the last 12 years, only a handful were at the hands of intruders, experts say. Now, just in the last month, at least five people have been killed and several wounded by intruders shooting in schools.

“This is like somebody coming to your front door, holding your house hostage and killing people,” said Ronald Stephens, director of the non-profit National School Safety Center in California.

“It’s whole different level of crime,” he said. “It means schools are so vulnerable that anyone can walk in off the street and do something so heinous. What is the next wave of shock we can get?”

Experts can only speculate as to motives.

Maybe they are copycat cases. Maybe schools are just easy targets. Maybe the incidents just reflect the violence of the society around them.

The shootings in the last month have occurred in small towns outside big cities or in rural areas where trust among residents is high and few precautions are taken at school sites.

“You have the what I call the `Mayberry Syndrome,'” said Jared Lewis, a Wisconsin author who has reviewed school shootings nationwide. “People think, `It can’t happen here.'”

Smaller schools such as the one-room schoolhouse in Pennsylvania rarely have the metal detectors or elaborate security found in schools in big cities.

Chicago Public Schools, for example, has required manned metal detectors in high schools and monitored entry points at elementary schools since 1996.

“Unfortunately, we are susceptible to the sorts of violence that happens in urban environments, so we’ve been ahead of the race for a long time when it comes to safety,” said Celeste Garrett, spokeswoman for Chicago Public Schools. “We have some of the safest learning environments around.”

But even in small school districts, there can be warning signs. In last week’s shooting in Colorado, the gunman was on campus the day before and was asking students questions, experts noted.

“The best preparation is a well-trained and highly alert staff and student body,” said Kenneth Trump, president of the National School Safety and Security Services consulting firm in Cleveland, which trains school in safety measures.

Still, Trump said: “In the case of a roaming monster walking around a school, you have to be realistic. There are some incidents you’re not going to be able to prevent.”

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etorriero@ tribune.com