(CNN) -- 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."
Experts in criminology can point to various mental, physical and situational factors that many shooters have in common, but most people who would fit into that box will never actually commit violent crimes.
Patterns in violent minds
In the United States, mass killers tend to be white males who perpetrate these acts in relatively well-to-do areas where violence is otherwise rare, places like Newtown, said Dr. Peter Ash, a forensic child and adolescent psychiatrist at Emory University School of Medicine.
Often, the killer has experienced chronic strain, depression or frustration over a long period, Levin said. In school, they were usually bullied, harassed and ignored.
The problems facing the men who commit these crimes are not uncommon, said James Garbarino, professor of psychology at Loyola University Chicago and author of "Lost Boys: Why Our Sons Turn Violent and How We Can Save Them." These include depression and anger about feelings of rejection and exclusion. Many of them have had some sort of mental health concern.
Dylan Bennet Klebold, who was one of the two high school seniors who participated in the 1999 Columbine High School shooting, appears to have battled depression. Klebold and Eric David Harris killed 13 people before they both committed suicide. Klebold wrote in his journal about suicide, according to the Jefferson County, Colorado, Sheriff's Department. In one entry, he wrote: "I swear -- like I'm an outcast, & everyone is conspiring against me. ..."
Harris appeared to be a psychopath, lacking empathy, Levin said. Harris wrote in his diary, according to the sheriff's department, "I'm full of hate and I love it." He wrote in his 1998 yearbook, on the pictures of almost every student, words including "worthless," "die" and "beat."
Still, people with mental problems generally are not automatically violent. A 2006 study in the American Journal of Psychiatry found that only 5% of all violent crimes in Sweden are committed by people with severe mental illness, which is likely to be similar in other countries such as the United States, the study said.
Russ Hanoman, a friend of Lanza's mother, described Lanza as "very withdrawn emotionally." He and other acquaintances reported Lanza's mother said that he had Asperger's syndrome, a diagnosis of high-functioning autism that CNN has not been able to confirm. There is no link between autism and planned violence, autism groups have noted.
Killers tend to blame others, not themselves, for their problems. Mass killers tend to target people whom they imagine would torment them, or whom they blame for their distress, Ash said. Why Lanza would target young children remains mysterious. Did he want to provoke outrage? Ash wonders. Is it because children seem happy and carefree?
"I believe he also sought to get even with society at large by killing the most cherished members: our children," Levin said.
Feeling alone, and with access to guns
Almost all the killers Levin has studied appeared to lack social support, having no one to turn to when in trouble. They reject their peers, and they are in turn rejected -- "alone in a psychological sense," Levin said.
Often the shooter has experienced a catastrophic loss, such as rejection from a girlfriend or getting fired from a job, Levin said. Parents may also inadvertently push teens and young adults over the edge by, for example, pressuring them to be successful.
Access to and training in the use of firearms is another major factor in executing a massacre like this, Levin said. The Bureau of Alcohol, Tobacco, Firearms and Explosives confirmed to CNN that Lanza and his mother frequented several gun ranges over the past several years. Those who knew the mother said she kept many weapons, including assault rifles and handguns, in her home. Investigators believe Lanza took his mother's guns to Sandy Hook Elementary School.
Although people have a constitutional right to gun ownership, they should not make them readily available to children who might be troubled or severely mentally ill, Levin said. School shootings in recent decades often happened with weapons taken from parents.
As much as mental health experts and criminologists may point to all these "warning signs" in the behavior of mass killers after a tragedy such as the Newtown shootings, the reality is that there are hundreds of thousands of people who fit that profile, Levin said. It's very hard to predict who will commit such a crime.
Planning for a crime
Carrying out planning rules out a diagnosis of psychosis, which is associated with a disconnect from reality, Garbarino said.
"The overwhelming majority of mass killers are not psychotic, and they're quite methodical in the way they commit an execution of the people they feel are responsible for their problems," Levin said.
Many shooters do appear to plan their attack over time, Ash said. From a passing thought, they become obsessed with revenge fantasies. They will read about previous shootings online; the websites they read may hint at what they're planning. From there, they move toward scoping out a location and deciding what they will wear.
That assassinations occur after a planning process like this is an insight from the U.S. Secret Service, Ash said. If agents come across someone who has been tracking the president's movements for six months, that's a much higher-risk situation than someone who just has an idea about attacking.
In the April 20, 1999, Columbine High School shootings, Harris and Klebold had clearly done significant planning. Officials found a piece of notebook paper "showing a diagram of the Columbine High School cafeteria with two X's next to the pillars," and a list of how many people were there before and after the first lunch period, the sheriff's department said. Both boys had written an itinerary for the day of the shooting.
The largest single school shooting in U.S. history, which occurred at Virginia Tech in 2007, was perpetrated by Seung-Hui Cho. Cho had been declared mentally ill and "an imminent danger" to himself by a Virginia special justice, CNN reported in 2007.
The massacre, in which Cho killed 32 people before turning the gun on himself, was not spur-of-the-moment, either. During the two hours between the two shootings he committed at different dormitories on April 16, 2007, Cho mailed a package with 27 videotaped messages and an 1,800-word statement to NBC News. "You forced me into a corner and gave me only one option," he said in one of the videos.
Schizophrenia is a diagnosis that has come up in some instances of mass killings -- for instance, Jared Loughner, who killed six people and wounded former U.S. Rep. Gabrielle Giffords outside an Arizona supermarket in 2011.
Ted Kaczynski, the "Unabomber," received a diagnosis of schizophrenia from court-appointed psychiatrists. This is an especially likely diagnosis in cases when the shooter is in his late teens or early 20s, because symptoms often do not develop until that age, Levin said.
But in other cases, the shooters' "pathology" is more clearly tied to their situations, Levin said. They have given up hope for the future, and they seek revenge. They believe they are the victims, and the people they are shooting are villains.
School shooters specifically tend to come from middle- or upper-class families, Garbarino said. In poorer areas already rife with violence, kids with the same vulnerabilities may drop out of school or end up in the criminal justice system much earlier than in a resource-rich community, so school itself might hold less meaning for them.
"For school shooters, they're sort of emotionally living and dying in high school," Garbarino said.
Kipland P. Kinkel, a freshman at Thurston High School in Springfield, Oregon, killed his parents and two students in his school's cafeteria in 1998. His parents were both teachers. His family had taken him to therapy, concerned about his obsession with guns and explosives.
"They got every possible resource, and it still didn't work," Garbarino said.
Garbarino interviewed a young man who had brought guns and bombs to school on Valentine's Day of his senior year of high school. The boy had studied the Columbine shooting and contemplated killing himself or his tormenters, who had been bullying him. Before he acted, a couple of girls saw him and found his behavior odd; one of them went to get a police officer. He surrendered and went to jail. He said in retrospect that he didn't want the girls to get hit if he started shooting.
School shootings by students dropped significantly after Columbine because schools took action to encourage everyone in their communities to bring threats to school officials' attention, Ash said.
"We've averted a number of mass killings at schools around the country in recent years because young people are beginning to inform when they hear one of their peers threatened in the hallway," Levin said. "They may inform a resource officer, a parent, a teacher, a school psychologist."
In the past, "It wasn't cool for teenagers to inform on their peers, and they didn't," he said. "More and more young people now recognize the seriousness of these threatening utterances at school, and they're much more likely to inform someone."
What have emerged instead are instances where members of the general public have taken up arms and killed innocent people.
Garbarino also points to a culture of gun violence, where killing people may seem like a viable and available solution to life's problems.
The good news is that most people who have these risk factors will never take the next step, Garbarino said. They may play violent video games, talk about violence with friends and research it on the Internet, but they would never implement a full plan.
A major problem is that often, interventions happen when a person becomes dangerous or threatening. A young person should receive help when he or she is merely "troubled," Levin said -- for instance, if he or she is being bullied and harassed, and feels a profound sense of powerlessness.
Anti-bullying laws can help, he said. Parents, teachers, principals and school psychologists should step in when they realize a young person feels terrorized in school, Levin said. Most school shooters had been bullied chronically; bullying is not necessarily just a part of growing up.
"When we see those red flags in the life of a youngster, we should intervene," Levin said. "Not to prevent a murder, but to do it because it's the right thing to do.
"We would improve the quality of life for lots of people," he added, "and in the process we probably would prevent a murder or two."