The massacre at Virginia Tech challenges some of our most basic assumptions: That schools are safe. That we are largely in control of our lives. That unpredictable and unexplainable horrors do not happen - not to most people.
Monday's events have left students on the Blacksburg campus reeling. Similar anguish is rippling through other campuses and among scores of parents who felt a rush of panic when they heard the news.
For the rest of us, the incident pricks at our comfort levels.
"It makes you think," said Robin Mancino of Downingtown. "If I'm just shopping or out anywhere, if someone has a grudge." She paused. "You just don't feel safe."
Usually, said Beth Berman, a child and adolescent psychologist at Albert Einstein Healthcare Network, "we get out of bed every day, and we just kind of live with an unspoken assumption that we'll get back in bed that night. We have an expectation of safety."
Statistically, that holds true. "Most instances of violence are one-on-one and perpetrated by people who we know," said Shawn Cahill, assistant professor of clinical psychology at the Hospital of the University of Pennsylvania.
That's hard to remember when a child is struck by a stray bullet in a city drug shooting. Harder still, perhaps, when more than 30 people are slain in one morning on a tight-knit campus.
Tell it to the parents and other loved ones who apparently frantically dialed their phones, trying to reach sons and daughters, brothers and sisters.
Between 9 a.m. and 1 p.m. Monday, usage on the Verizon Wireless network spiked fourfold. Officials tracking Verizon's land lines said the switching station handling calls to and from the Blacksburg area had twice the normal volume.
Yesterday at Muhlenberg College, a student told social psychologist Kathleen Harring's class that the windows of her first-floor dorm room are barred. Before, that seemed like a good idea. Now, she wonders what she would do if she had to get out.
Events like those at Virginia Tech and so many others - the Amish school shootings, Columbine, the 9/11 attacks - show us that bad things really can happen.
But not to most people.
"The issue is the uncontrolled, unpredictability of what happened," said Anthony Mannarino, a psychologist on the Pittsburgh campus of Drexel University College of Medicine. "Those kinds of things are very scary and disturbing."
When Robert L. Sadoff's four children were in school, he used to think how wonderful it was that they were safe inside, busy learning.
"We used to have safe havens, but where are they now?" said Sadoff, a clinical professor of psychiatry at the University of Pennsylvania.
Even the well-protected Green Zone in Baghdad wasn't safe enough, he said.
Experts and parents alike worry that our culture is steeped in violence - in Iraq, on TV, on Philadelphia's very streets.
"Some might argue we're getting numb to it," Berman said. "I think we're getting more anxious all the time."
Worse, a horror like the Virginia Tech shootings often can never be explained in a way that makes sense.
"For no known reason -" began Kim Giannini, a Chester County resident with a 4-year-old son and a 9-year-old daughter. "Why someone can snap that quickly -" She trailed off in dismay.
Maybe, said Sadoff, a forensic pathologist for more than 40 years, "you have to say: 'OK, this is one of the risks of being in the 21st century, where people have stepped on rules and feel that anything goes.' "
"It's a complex social, medical, psychological, economic issue," he said. "We have the war on drugs, the war on this, the war on that. I think we have to have a war on violence."
Then he laughed bitterly and corrected himself. "Maybe a peace on violence."
Berman hopes that through all their heartache and worry, people acknowledge their humanity and the goodness that's also a part of life.
"I'm sure," she said, "that random acts of kindness happen on that campus every day."
For most people.