In the past, "It wasn't cool for teenagers to inform on their peers, and they didn't," he said. "More and more young people now recognize the seriousness of these threatening utterances at school, and they're much more likely to inform someone."
What have emerged instead are instances where members of the general public have taken up arms and killed innocent people.
Garbarino also points to a culture of gun violence, where killing people may seem like a viable and available solution to life's problems.
The good news is that most people who have these risk factors will never take the next step, Garbarino said. They may play violent video games, talk about violence with friends and research it on the Internet, but they would never implement a full plan.
A major problem is that often, interventions happen when a person becomes dangerous or threatening. A young person should receive help when he or she is merely "troubled," Levin said -- for instance, if he or she is being bullied and harassed, and feels a profound sense of powerlessness.
Anti-bullying laws can help, he said. Parents, teachers, principals and school psychologists should step in when they realize a young person feels terrorized in school, Levin said. Most school shooters had been bullied chronically; bullying is not necessarily just a part of growing up.
"When we see those red flags in the life of a youngster, we should intervene," Levin said. "Not to prevent a murder, but to do it because it's the right thing to do.
"We would improve the quality of life for lots of people," he added, "and in the process we probably would prevent a murder or two."