Although some news reports have connected Lanza with Asperger's syndrome, there's nothing officially known at this point about whether Lanza had Asperger's, let alone whether it would have played any role in the killings. With the hope of gaining some insight into Lanza's motives, state officials have invited a geneticist to study the gunman's biology.

After a tragedy like the Newtown incident, it's natural to want quick answers as to why and how to prevent another one. But it doesn't work like that, UConn's Lewis said. The circumstances that create a mass murderer are multiple, she said, and they change with each incident.

Depressed? Not Necessarily

"Mental health professionals are not able to predict with any certainty violent acts," Lewis said. "After a tragic incident, people look back in hindsight and believe it should have been easy to foretell what happened. Although school shootings are tragic and have a high magnitude, they're rare, so it's difficult to predict."

Regarding long-term depression, Lewis said, "It is one of the variables that we consider." But she cautions against assuming that any school shooter who plans on being killed or committing suicide is depressed. "They could be really, really mad," she said, or have planned their own deaths as "going out in a blaze of glory."

And when a potential killer is socially withdrawn, danger signs are few and easy to overlook. But as Lewis noted, echoing Schwartz, the signs are there if you look hard enough.

"The adolescent, either on purpose or not, lets out information about what they're feeling and even their plans before they've murdered," she said, adding that in many cases, young people have explicitly boasted of the plans ahead of time, only to have no one take them seriously.

Nadine Kaslow, a professor of psychiatry at Emory University in Atlanta, said that identifying and getting help for socially withdrawn youths gets trickier as they get older, when schools focus more on academics than on overall behavior.

"It is more challenging in high school," she said. "There are all these demands academically and less attention is being paid to mental and emotional health. … One of the things I think that's important is to have more review meetings to look at how each [student] is doing, not just how they're doing academically, but how they're doing emotionally."

But even when safeguards seem to be operating, Schwartz said, it often fails to prevent violence. He pointed to the Aurora, Colo., movie theater shootings earlier this year in which 12 people were killed and the 2007 carnage at Virginia Tech that left 32 dead.

"In Aurora, that young man had been in the care of a psychiatrist and she notified security or police that she was very concerned about him," he said. "The Virginia Tech shooter had been identified and given a court order — that had fallen through the cracks."

"There are limits to what we can do."