Today's tragedy comes on the heels of that other mass shooting 72 hours ago in the mall filled with holiday shoppers outside portland. This singular tragedy in newtown, joining the ranks of the other... See More
Today's tragedy comes on the heels of that other mass shooting 72 hours ago in the mall filled with holiday shoppers outside portland. This singular tragedy in newtown, joining the ranks of the other dark days, coe, tus an, aurora. Pierre thomas now tells us what the experts say they see when they look at the men behind these massacres. Reporter: The virginia tech A shooting spree in tucson, arizona --six dead. 12 killed at a colorado movie theater. Mass murder conducted by a lone gunmen. But who are these men? And what motivates them? We're talking about attacks that are carried out largely by people who are not necessarily psychopathic, not sociopath irk, but folks who are relatively normal, but at a point of absolute desperation. Reporter: This year alone, there have been 15 mass shootings with at least 84 people killed before today. In the 61 mass murders carried out since 1982, all but one of the 43 shooters were male. Most were white and the average age of the primary shooter was 35.Ly half of the mass murders have involved school or workplace shootings. But in recent years, shopping malls, restaurants, and houses of worship have also come under fire. Moose shooters tend to look for soft targets because they're easy. They want to be able to walk into a situation where people are not armed, they have no idea what's about to happen to them. You can get the jump on them. Reporter: In many cases, the shooter targets specific people for specific reasons. Sometimes they want fame and notoriety. Sometimes they see it as a way to solve a problem or get back at someone, some form of revenge. Reporter: And often, as we have seen at virginia tech, tucson, and colorado -- the shooters have serious mental health issues. Usually the attackers will tell other people beforehand what they're thinking about and planning to do. Reporter: They often exhibit warning signs. Some are blatant --- other signs are more subtle and only become clear in hindsight. Diane, there are an estimated 200 million-plus guns already in circulation in the u.S. The fact is disturbed people are going to get them. One source predicted tonight we're going to see this sad situation unfold again. It may be days, it may be weeks or months, but he said, it will happen again.
This transcript has been automatically generated and may not be 100% accurate.