School shootings have become tragically familiar -- but no less disturbing and no less painful.
And according to psychologists, the violence that sparked the recent school shootings around the country might not stem from anything new. It's usually been bubbling below the surface all along.
"People snap when their ability to cope is overwhelmed," said Dr. Robert Trestman, director of the Center for Correctional Mental Health Services Research at the University of Connecticut Health Center.
But how much stress or frustration people can cope with?
"For some, minor stressors add up," producing a "hassle factor," said Trestman. But for others, one overwhelming event can send a person over the edge.
"In our world, we have not structured the community supports and skills training for kids to develop adequate, flexible coping strategies," said Trestman.
But as recent schoolyard tragedies have illustrated, it's not just students who snapped. On Aug. 24, a 27-year-old man walked into an elementary school in Essex, Vt., and killed two teachers and wounded two other people before shooting himself. He survived.
Last week, a 53-year-old homeless man walked into a school and molested several girls before killing one girl and himself in Bailey, Colo. On Monday, a 32-year-old milk truck driver lined Amish schoolchildren up against the blackboard of their one-room schoolhouse and shot five young girls before turning the gun on himself.
Unaware as communities may be, experts emphasize that this violence does not come out of nowhere.
"All our research has found that both adolescent and adult murderers plan these events over days, weeks and sometimes months," said Reid Meloy, a forensic psychologist for the Forensic Mental Health Division for San Diego County.
"But a humiliating event does typically put the plan into action, which will then unfold over the course of the next day or so," Meloy said.
Still, experts agree, sometimes there are signs.
People may give warnings, or talk about getting even. Students may talk about bringing guns to school, experts said.
But when the killers live outside a community, that community -- whether it's a school or the Amish community -- may be unaware of any lurking dangers.
In the case of killers who murder innocent students before killing themselves, "we may never know," said Katherine Newman, professor of sociology and public affairs at Princeton University, and author of the 2004 book "Rampage: The Social Roots of School Shootings."
"And no answers will ever take the tragedy away."