Many people who knew Adam Lanza said he was awkward, withdrawn and socially isolated. It's a common description of mass shooters, but it also describes countless numbers of young people, most of whom grow out of their shyness and go on to lead normal lives.

The challenge for mental health experts is identifying — out of the sea of awkward youths — when a social disconnection signals danger. How do you identify the potential mass murderer?

"Here, we're out at the leading edge of information, where nobody knows," said Dr. Harold Schwartz, psychiatrist-in-chief at The Institute of Living, a mental health center in Hartford. "When you think of the number of kids who have committed acts like this and have survived to tell us about them, there have been really very few. I don't think anyone can say what happens in their minds."

Schwartz and other mental health experts this week said a big problem is that the term "loner" fits so many people and doesn't begin to explain how someone can commit the kind of unimaginable violence of mass murder.

"There are so many people in the world who fit the description of the awkward loner who's socially disconnected — you'd have to screen hundreds of thousands of people," he said. "The question is, 'How good are we about detecting the signs and symptoms?' And, clearly, we could do a lot better."

But, he said, there are some characteristic patterns — "grievance collectors," for instance.

"One major factor in these folks is this notion of collecting grievances and harboring them and having them fester in some fantastic, twisted way," Schwartz said. "And some people who harbor their anger have no productive way to come to a resolution, so the anger just grows and they tend to project outward on others as the source of the grievances."

"Through some fantastic thinking, they come to believe, 'I can show the world how much I hurt, and then everyone will understand, and they will be sorry,'" Schwartz said.

Self-Centered To The Extreme

Those who have a predisposition to committing horrific acts of violence tend to have an extreme form of self-centeredness known as solipsism, Schwartz said.

"We are all of us on some continuum of connectedness to others," he said. "To them, they are the only one really living, and their thinking is that everyone else is a cardboard fixture there to support them."

Social isolation exacerbates that sort of thinking and creates a kind of spiral.

"The more time you spend in solitude in your room, looking at stuff on the computer and playing violent video games," he said, the more tenuous a person's connectedness to others becomes. That can break down any innate "prohibition against killing others," Schwartz said.

Dr. Catherine Lewis, a forensic psychiatrist at the University of Connecticut, said it was important to distinguish between kids who simply don't have many friends and those who have been rejected — both categories often covered by the label of "loner."

"It's a word used a lot to describe people who commit shootings, but there's a big difference between someone who is happy to be solitary and someone who, by their own difficulties, is not accepted," she said. "The issue is more of being a failed joiner, rather than being a loner."

And for some people who have been ostracized by their peers, said Alan Kazdin, director of the Yale Parenting Center and Child Conduct Clinic, the response is to conform as much as possible to the group.

"When that doesn't work — and it usually doesn't — they stop that and turn away and become aggressive," he said.

Although "loner" is a common descriptor of mass shooters, it hardly fits them all.

A 2011 study led by Christopher J. Ferguson, an assistant professor of psychology at Texas A&M International University, notes that of 41 school shooters from 1970 to 2000, only 34 percent could be accurately described as "loners." However, 71 percent "perceived themselves as wronged, bullied, or persecuted by others."

Other findings featured in the study include: Few of the shooters had been diagnosed with mental health problems or received mental health services, despite 78 percent of them having experienced suicidal attempts or thoughts and 61 percent having had a history of significant depression.