What Were They Thinking?

School shootings like the one at Northern Illinois University are usually linked to a mental disorder of the gunman, whose anger, social isolation or desire for attention bursts into violence, experts told the ABC News Law & Justice Unit.

Some of those experts recommend that schools make mental health a criterion for admissions, while others say steps as simple as locking classroom doors go a long way toward safer schools. And while media coverage no doubt prompts some "copycat" incidents, the experts say news coverage also provides important information that can make prevention possible.

Why Do School Shootings Happen?

A mental health problem is often involved, experts said. The NIU gunman, for example, reportedly stopped taking medication for an unspecified disorder and was allegedly behaving erratically in recent weeks. An inordinate drive for attention can lead shooters to create a spectacle, to copy incidents like the Virginia Tech shooting, which drew enormous publicity, says Katherine Newman, a professor at Princeton University. Randomly shooting strangers is more about crafting a dangerous image than about the victims, she said.

The most recent shootings have generally occurred on college campuses because shooters in college are older and their mental disorders are further along, Newman said. Colllege age shooters are less constrained by the rules and social structure of high school and college campuses are physically more open and harder to monitor, she told ABC News.

The Columbine High School massacre in 1999 was a watershed moment, said Ron Stephens of the National School Safety Center. It provoked copycat shootings and illustrated the possibilities of directing deadly violence at not just at an individual, but indiscriminate targets.

How Can They Be Prevented?

Stephens recommended that colleges and selective high schools evaluate students on their mental health as well as their academic and athletic talent. He acknowledged the legal obstacles to doing so — a slew of privacy and anti-discrimination laws — but sees an analogy in how employers ask prospective employees to sign releases that allow background checks and examination of confidential information. Stephens said he believe it's a serious enough problem to change the laws, if necessary.

High schools are generally much better than colleges at preventing and managing shootings because they have more efficient systems for getting information out to thousands of students and teachers, said Michael Dorn, a school security expert at Safe Havens International. Dorn said he believes basic steps like require teachers to lock their classrooms in the event of a school emergency could save lives. Several experts pointed out that NIU apparently took steps to prevent or at least contain school shootings.

Following the Virginia Tech shooting, NIU launched a website to broadcast emergency updates and incorporated the capacity for instant messaging into their emergency strategy, said Kenneth Trump, president of National School Safety and Security Services.

"Somebody learned their lessons from Virginia Tech," he told ABC News.

What Role Does the Media Play?

Certainly the media draw attention to the gunmen and to some extent encourage copycats, experts said. The media can make shootings seem like "business as usual," Newman noted, and extensive news coverage may explain why shootings "tend to come in clusters," added Hill Walker, co-director of the Institution on Violence and Destructive Behavior at the University of Oregon. But if the media didn't alert its audiences to mass shootings, the public wouldn't know how to look for and interpret the warning signs of a potential shooter, Newman said.

"It's just better if everyone is informed," Stephens said.

The Worst Incident of School Violence

Columbine? Virginia Tech? It was actually a 1958 fire that burned down Our Lady of Angels, a Catholic school in Chicago, and killed 92 students. One or possibly two fourth graders set the fire. The nuns told the students to pray rather than escape, Dorn said.