"There's usually some precipitating event," she tells correspondent Maureen Maher. "It might be a loss of status, like a public humiliation that comes from bullying. But the important thing for the shooter, it feels like a major loss. Something they can't overcome."
Randazzo says all of the 41 shooters they studied had suicidal thoughts before carrying out their attacks. "As we see it, there's really just a fine line between suicide and homicide. They actually feel 'Okay, I'm desperate. But who's to blame for this? Why am I feeling such despair?'" Randazzo explains. "And they may realize, 'Okay it's because of that teacher, that student, because I was bullied, and I'm going to take some people out before I kill myself.
In November 1985, then 17-year-old Jamie Rouse walked into his Lynnville, Tenn. high school with a specific target in mind.
"I was under a lot of stress. I guess I blamed school and the teachers and I was just so full of hate and anger and this evil that I guess I just felt that they should be punished, too," Rouse has said.
But what happened next was surprisingly typical of most school shootings, according to Randazzo. After Rouse shot his first victim, the killing continued.
"When you're in that state of mind where you're going to kill someone, nothing matters. I wasn't expecting to come back alive," Rouse said.
That's exactly what happened in 1997 in Pearl, Miss., after 16-year-old Luke Woodham killed his ex-girlfriend and her best friend.
"He shot them both when he got into school. But then he starting shooting a number of other students," Randazzo explains.
In 1998, Kip Kinkle killed both his parents and then continued his shooting rampage at his Oregon high school.
Click here for an interactive gallery of the victims.
And in the deadliest school shooting ever, until Monday's incident, Charles Wittman killed his mother and wife, before ending the lives of another 14 victims at the University of Texas.
Not only are many of these school shootings planned well in advance, the killers often confess their plans before committing the crime. "These are people who usually tell other kids what they're planning to do," Randazzo says.
Jamie Rouse, who is now serving a life sentence, told five of his friends and, in fact, one of them even drove him, with gun in hand, to Richland High School that day.
"I remember him making some comment, 'So you're really going to do it aren't you?'" Rouse remembered.
"There's information out there before a school shooting occurs. The challenge for schools is how to encourage them to share that info with adults," Randazzo says.
Even though adults may not hear of the murder plans in advance, Randazzo says many are witnessing the danger signs. Her study found that in the vast majority of these shootings, at least three adults had witnessed suspicious behavior by the students, but did nothing about it.
"The kids from Columbine had been on the radar screen of law enforcement for having a very threatening Web site at least a year in advance of the Columbine incident," Randazzo says.
But since the Columbine killings, Randazzo says adults have become more aware. "One of the things we've seen high schools do really well is they've taken a very proactive approach to investigate rumors and really address concerning behaviors among students. What we're trying to do is prevention," she tells Maher.
In one success story, the staff at one high school trained its faculty to spend just a few minutes each day chatting with their students. "They started to get back a wealth of information," Randazzo explains. "They heard about drug use, who had brought weapons to school, who's getting beat up at recess. They started to learn so much more, just because they were stopping students and asking how they were doing."
Across the country, authorities have seen improved communications pay off, and since Columbine more than a dozen school shootings have been thwarted while still in the planning stages.
But all preventive measures aside, Randazzo says, in the end, America may be its own worst enemy. "What I really see as the distinguishing factor is that in this country it is very, very easy to get access to weapons," she says. "In other countries they have the same dynamics but without that access to lethality, so you see kids getting into fist fights and knife fights, and they may kill themselves, but you don't see the magnitude of destruction that we have seen in this country and certainly what we saw yesterday at Virginia Tech."