If you think what happened at Virginia Tech is incomprehensible, you're about to meet some people who understand that kind of madness very well: they're the people who protect the president of the United States.
For years, the U.S. Secret Service has sent psychologists into prisons and mental hospitals to interview those bent on assassination.
As Scott Pelley reports, their interviews bring extraordinary insight into the mind of an assassin; what has been discovered in the process is that many of the same characteristics found in assassins can also be found in school shooters.
In recent years, 60 Minutes has had unprecedented access to the Secret Service Intelligence Division.
When it comes to protecting the presidential motorcade, the Secret Service training center outside Washington D.C. is without a doubt the most hostile "town" in America. You won't find its streets on any map; the center was built after the Kennedy assassination to drill the agents of the presidential protection detail. There's even an airport with one half of an Air Force One plane, where agents take turns playing the commander-in-chief and the gunman in the crowd.
This is practice for last ditch defense, but as one agent told 60 Minutes, "If the guns come out, we've already failed."
It's up to the Secret Service Intelligence Division to stop the assassin before he picks up a gun. They open new cases every month, investigating people who may want to harm those under Secret Service protection. The trouble is how to sort out those who just make a threat from those who actually pose a threat.
"Many of those who committed attacks did not threaten prior to their attack of violence," explains former Special Agent Brian Vossekuil.
In 1999, Vossekuil and psychologist Dr. Robert Fein were the primary authors of a groundbreaking Secret Service study of stalkers and assassins. They called it the "Exceptional Case Study."
They analyzed 83 attacks, and interviewed gunmen including Arthur Bremmer, who gunned down presidential candidate George Wallace, and Mark Chapman, who murdered John Lennon.
"What was it that struck you about these 83 cases you researched in the exceptional case study?" Pelley asks Dr. Fein.
"There was no, 'quote' profile of an assassin or a near assassin. People came from a range of backgrounds. Some had criminal records, most did not, some had histories of violence, most did not," Fein explains.
"The behavior in the acts generally included, things like communication to others, planning, target selection," Vossekuil explains.
"These were not impulsive, out of the blue, attacks. They were part of a process," Fein says.
"And we found, as Robert just said, acts that were in engaged in that was identifiable, understandable and consistent with someone on might be on a pathway toward mounting an attack," Vossekuil adds.
In one of their interviews, in a psychiatric ward, Vossekuil and Fein talked to a man called "J.D."
"I was looking for a location where I could test fire the gun," J.D. told the researchers.
In the late 1980's, J.D. stalked two presidents across the country, robbing banks to pay for the travel. What was his motivation?
"J.D. was a person who had dropped out of graduate school, who had served in the military, who became convinced that he had a choice to make, that aliens were ordering him either to kill innocent schoolchildren or to kill the president," Fein explains.
"He sounds too crazy to be a threat," Pelley remarks.
"Because he was quite organized, because he believed that he had this horrible choice. And the organization that he had to look normal, to explore security, to get weapons, to travel around the country that was quite chilling. Though if you talked with him, he was – he did not come across as a hostile, angry – fitting any stereotype of quote an assassin," Fein says.