The scene is unfortunately familiar to many Americans: students hugging each other outside of school. Victims being rushed to a hospital. Police tape surrounding a school.

There have been more than 50 school shootings in the United States since 1996. Almost all of the shooters were students or former students. Teachers, principals and other students have died in violence that shocked communities all over the country.

In late February, it happened in tiny Chardon, Ohio.

"At approximately 8:00 a.m. this morning, a Weston High School custodian observed a 15- year-old student enter the high school carrying a shotgun," an official announced.

The gunman killed three students. The alleged shooter: a 17-year-old student named T.J. Lane.

Psychologists, police and educators have searched for shared traits among the shooters. A U.S. Secret Service study found school shooters had almost nothing in common...except that most of them were boys, usually students at the school. And most acted alone.

"Three quarters of them reported feeling bullied and persecuted, ostracized by their peers," explained Psychologist Amanda Nickerson, who directs the Alberti Center for the Prevention of Bullying Abuse and School Violence at the University at Buffalo.

"Often, after the fact, we find out that they were depressed and/or suicidal," Nickerson added. "And they also had some major stressor or perceived loss that may have triggered what happened."

Mental illness also plays a critical role.

"When we look into the histories of people who have acted violently, we find a pattern of mental illness," Nickerson stated.

But getting help can be difficult. War Eagle Eastwood said his son reported hearing voices and seemed confused before taking a high-powered rifle to a Colorado school in 2010.

"He tried to get help, and he said he went to a few places and that, but they told him if you haven't got the money to pay for this, we can't help you for it." Eastwood said.

Professor Nickerson says vigilance is the best precaution. "Most of the time it's not just a random act. There's some evidence that other people knew about the plans or that there was planning in advance," she said.

The Secret Service report shows someone, usually another student...sometimes several students...knew about the attacker's plan before it took place, but they didn't tell authorities.

School shootings are relatively rare, which is why they get a lot of media coverage. The Secret Service estimates that a student's chance of being killed in school violence is about one in a million. But that's little comfort to the families mourning their loved ones killed at a place where they should have been safe.

"In the space of a minute, our whole world is turned upside down, a Priest noted. "And nothing can prepare us for something like this." .

T.J. Lane is scheduled for a hearing on April 3.