They are cruelest for their innocent settings – school campuses and restaurants – and each record-breaking shooting rampage poses a twisted challenge to a future mass murderer.

Until a gunman killed 32 people Monday at Virginia Tech, the most people slain in a single massacre were the 23 murdered at a Luby’s Cafeteria in Texas in 1991 and 21 at a California McDonald’s in 1984.

Mass murderers choose those types of spots, where they are bound to find many victims, for their lax or nonexistent security, sometimes with the intention of outdoing the last bloodbath.

“Some of it is copycat and wanting to be more sensational than the last one,” said John Morgan, a retired chief of police in Arkansas and Florida who now works at the School Violence Resource Center in Little Rock.

Although the number of casualties is not climbing with each successive rampage, access to semiautomatic weapons helps shooters take out many victims at once, said James Fox, a professor of criminal justice at Northeastern University.

“Obviously, the nature of the weaponry being used is particularly powerful.

“It’s hard to kill so many people with a knife or even a pistol,” Fox said.

The very public nature of campuses, restaurants and sometimes workplaces creates a pool of sitting ducks.

Colleges and universities are especially vulnerable because of the difficulty controlling their sprawling, high-traffic environments, Morgan said.

“It takes place at restaurants and schools for an obvious reason: There’s a lot of people together. It’s an easy place to commit a mass murder,” said Louis Schlesinger, a professor of forensic psychology at John Jay College of Criminal Justice.

He and others are troubled by the implications of Monday’s death toll.

“I hate so much discussion about it being a record,” Fox said. “It just challenges more people to break the record.”

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