Random acts of violence: What makes someone go on a shooting spree?

A 20-year-old man walks into a Tacoma mall and indiscriminately starts shooting; two teens killed 13 classmates at Columbine high school before taking their own lives; a 37-year-old man killed six people on a New York commuter train. All horrific crimes, committed with a chilling degree of randomness that shatters the perception of safety in public spaces.

They're given ominous names like 'mass murderer' or 'rampage shooter' and they become the subjects of movies of the week. While these killings make for sensational news coverage, the deeper rationale behind such an act often gets glossed over by body counts and gruesome images.

James Alan Fox, a professor of criminal justice at Northeastern University, is an expert in the psychology of the criminals behind these shooting sprees. He says the shooters are usually loners who blame the world for their problems.

"Rampage shooters hold a grudge against society -- no one in particular, that society is unfair, that he's a victim," Fox said. "Generally they do see themselves as victims. This is an attempt to get even with as many people as possible and it doesn't really matter who."

In the case of Dominick Sergio Maldonado, the man who shot six people, one critically, at a mall Sunday in Tacoma, Wash, what struck Fox was his age -- 20. Usually, it takes time for "one's coping mechanisms to wear down and the level of anger to grow to the point where an event like this can happen."

The author of "Extreme Killing: Understanding Serial and Mass Murder," Fox specializes in mass killings that have four or more victims, a crime that occurs "a couple of dozen times a year."