If you’re wondering who else in the United States might fit a “profile” of becoming a mass killer, just look around: They are everywhere, and they’re most likely harmless.

Frighteningly, we have little idea about what separates those who ponder committing slaughter from those who go through with it. Experts say that risk factors, such as social isolation and rejection, are found in many people across the United States, a country shaken by the massacre at Sandy Hook Elementary School in Newtown, Connecticut, on Friday, in which 20 children and six adults were killed.

Adam Lanza, 20, the shooter, whom police say turned the gun on himself Friday, attended the school. He has been described as quiet and socially awkward. There are still few details of what could have motivated his actions.

“The truth is that there are many people who have all the symptoms, and don’t get the disease,” said Jack Levin, professor of sociology and criminology at Northeastern University in Boston. “They may be loners, and strange and angry and have access to firearms, but they don’t hurt anyone.”