Story highlights Experts: Mass killers tend to blame others, not themselves, for their problems

They lack social support, having no one to turn to when in trouble

Often the shooter has experienced a catastrophic loss

Most people who fit this profile will never kill anyone

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.

Photos: Funerals: Community says goodbye Photos: Funerals: Community says goodbye Newtown funerals: Community says goodbye – Mourners wipe tears away as they file out of the Church of Jesus Christ of Latter-day Saints after the funeral of Emilie Parker in Ogden, Utah, on Saturday, December 22. Hide Caption 1 of 30 Photos: Funerals: Community says goodbye Newtown funerals: Community says goodbye – The casket of Rachel Marie D'Avino is carried into the Church of the Nativity in Bethlehem, Connecticut on Friday, December 21. Hide Caption 2 of 30 Photos: Funerals: Community says goodbye Newtown funerals: Community says goodbye – Parents attend the funeral of Dylan Hockley, 6, a victim of the Sandy Hook Elementary School shooting, on December 21 in Newtown, Connecticut. Hide Caption 3 of 30 Photos: Funerals: Community says goodbye Newtown funerals: Community says goodbye – A view from outside the funeral service for 6-year-old Dylan Hockley at Walnut Hill Community Church on December 21. Hide Caption 4 of 30 Photos: Funerals: Community says goodbye Newtown funerals: Community says goodbye – Family members depart the Trinity Episcopal Church in Newtown, Connecticut, following a funeral for Benjamin Wheeler, 6, on December 20. Hide Caption 5 of 30 Photos: Funerals: Community says goodbye Newtown funerals: Community says goodbye – Mourners, including Boy Scout and Tiger Scout members, depart the funeral on December 20, 2012. Wheeler was a member of Tiger Scout Den 6. Hide Caption 6 of 30 Photos: Funerals: Community says goodbye Newtown funerals: Community says goodbye – Boy scouts salute as a funeral procession for Benjamin Wheeler enters the Trinity Episcopal Church on December 20, in Newtown, Connecticut. Hide Caption 7 of 30 Photos: Funerals: Community says goodbye Newtown funerals: Community says goodbye – Firefighters salute as the casket of Daniel Barden, 7, a victim of the shooting at Sandy Hook Elementary School, is removed from St. Rose of Lima Church on Wednesday, December 19, 2012, in Newtown, Connecticut. Hide Caption 8 of 30 Photos: Funerals: Community says goodbye Newtown funerals: Community says goodbye – Police escort the hearse bearing the casket of Daniel Barden from St. Rose of Lima Church on December 19. Hide Caption 9 of 30 Photos: Funerals: Community says goodbye Newtown funerals: Community says goodbye – Mourners enter St. Rose of Lima Church for the funeral of Charlotte Bacon, 6, on December 19, in Newtown. Hide Caption 10 of 30 Photos: Funerals: Community says goodbye Newtown funerals: Community says goodbye – A police officer directs traffic as mourners enter the church for Charlotte Bacon's funeral on December 19. Hide Caption 11 of 30 Photos: Funerals: Community says goodbye Newtown funerals: Community says goodbye – A procession arrives for the funeral of Victoria Soto, 27, at Lordship Community Church in Stratford, Connecticut, on December 19. Soto was a first-grade teacher being hailed as a hero for protecting the children in her class during last week's school massacre in Newtown. Hide Caption 12 of 30 Photos: Funerals: Community says goodbye Newtown funerals: Community says goodbye – A woman carries a program with Soto's photo after attending a funeral for the slain teacher in Stratford on December 19. Hide Caption 13 of 30 Photos: Funerals: Community says goodbye Newtown funerals: Community says goodbye – Bagpipers play at funeral services for Soto on December 19 in Stratford. Hide Caption 14 of 30 Photos: Funerals: Community says goodbye Newtown funerals: Community says goodbye – Richard and Krista Rekos leave after a funeral service for their 6-year-old daughter, Jessica, at Saint Rose of Lima Church on Tuesday, December 18, in Newtown. Jessica was one of 20 children killed in last week's school shooting. Hide Caption 15 of 30 Photos: Funerals: Community says goodbye Newtown funerals: Community says goodbye – Family and friends depart Jessica's funeral on December 18 in Newtown. Hide Caption 16 of 30 Photos: Funerals: Community says goodbye Newtown funerals: Community says goodbye – A child stands next to a makeshift memorial for Jessica Rekos following her funeral on December 18. Hide Caption 17 of 30 Photos: Funerals: Community says goodbye Newtown funerals: Community says goodbye – Pallbearers carry out James Mattioli's casket at St. Rose of Lima Roman Catholic Church after a funeral Mass on December 18 in Newtown. James, 6, was one of the 26 victims in the shooting at Sandy Hook Elementary School in Newtown. Hide Caption 18 of 30 Photos: Funerals: Community says goodbye Newtown funerals: Community says goodbye – Mourners console each other after the funeral for James Mattioli on December 18. Hide Caption 19 of 30 Photos: Funerals: Community says goodbye Newtown funerals: Community says goodbye – People arrive for the funeral of Jessica Rekos, 6, at St. Rose of Lima Roman Catholic Church in Newtown on December 18. Hide Caption 20 of 30 Photos: Funerals: Community says goodbye Newtown funerals: Community says goodbye – Jessica Rekos' casket arrives at St. Rose of Lima Roman Catholic Church as mourners gather December 18. Hide Caption 21 of 30 Photos: Funerals: Community says goodbye Newtown funerals: Community says goodbye – Mourners console each other after attending the funeral for Jack Pinto, 6, on December 17. Hide Caption 22 of 30 Photos: Funerals: Community says goodbye Newtown funerals: Community says goodbye – Mourners grieve the death of Jack Pinto, 6, on December 17. Hide Caption 23 of 30 Photos: Funerals: Community says goodbye Newtown funerals: Community says goodbye – A mother and two children attend the funeral for Jack Pinto on December 17. Children are among those crowding the funeral for the 6-year-old boy. Hide Caption 24 of 30 Photos: Funerals: Community says goodbye Newtown funerals: Community says goodbye – A man comforts a young mourner at Honan Funeral Home while attending the funeral for Jack Pinto, 6, on December 17. Hide Caption 25 of 30 Photos: Funerals: Community says goodbye Newtown funerals: Community says goodbye – Boys enter Honan Funeral Home before Jack Pinto's funeral on December 17 in Newtown. Hide Caption 26 of 30 Photos: Funerals: Community says goodbye Newtown funerals: Community says goodbye – Veronika Pozner, mother of Noah Pozner, arrives for her son's funeral on Monday, December 17, at the Abraham L. Green and Son Funeral Home in Fairfield, Connecticut. Monday is the first day of funerals for the 20 children and seven adults who were killed by 20-year-old Adam Lanza on December 14. Hide Caption 27 of 30 Photos: Funerals: Community says goodbye Newtown funerals: Community says goodbye – Three women embrace as they arrive for the funeral services for Noah Pozner on December 17. Hide Caption 28 of 30 Photos: Funerals: Community says goodbye Newtown funerals: Community says goodbye – Mourners arrive at Noah Pozner's service in Fairfield on December 17. Hide Caption 29 of 30 Photos: Funerals: Community says goodbye Newtown funerals: Community says goodbye – Mourners leave Noah Pozner's service on December 17 in Fairfield. Hide Caption 30 of 30

Photos: Photos: Newtown school shooting Photos: Photos: Newtown school shooting Newtown school shooting – Children in Newtown, Connecticut, return to school on December 18, 2012, four days after the shooting at Sandy Hook Elementary School. Hide Caption 1 of 43 Photos: Photos: Newtown school shooting Newtown school shooting – Connecticut State Police officers search outside St. Rose of Lima Roman Catholic Church in Newtown, Connecticut, on Tuesday, December 16, after a threat prompted authorities to evacuate the building. Investigators found nothing to substantiate the reported threat, a police official said, declining to provide additional details. The church held Sunday services following the mass shooting at Sandy Hook Elementary School in Newtown. Hide Caption 2 of 43 Photos: Photos: Newtown school shooting Newtown school shooting – Connecticut State Police officers walk out of St. Rose of Lima Roman Catholic Church after the Newtown church received a threat December 16. Hide Caption 3 of 43 Photos: Photos: Newtown school shooting Newtown school shooting – Firefighters attach black bunting to a fire truck as a memorial at the fire station down the street from the Sandy Hook Elementary School in Newtown, Connecticut, on Saturday, December 15. Hide Caption 4 of 43 Photos: Photos: Newtown school shooting Newtown school shooting – Connecticut Chief Medical Examiner H. Wayne Carver II talks to the media about the elementary school shooting during a press conference at Treadwell Memorial Park on December 15. Hide Caption 5 of 43 Photos: Photos: Newtown school shooting Newtown school shooting – Zulma Sein is hugged by a family member outside of the entrance to the Sandy Hook School on Saturday. Hide Caption 6 of 43 Photos: Photos: Newtown school shooting Newtown school shooting – Police officers keep guard at the entrance to the street leading to the Sandy Hook Elementary School on Saturday, December 15. Hide Caption 7 of 43 Photos: Photos: Newtown school shooting Newtown school shooting – Connecticut State Police Lt. Paul Vance addresses the press on December 15. Hide Caption 8 of 43 Photos: Photos: Newtown school shooting Newtown school shooting – Police officers stand at the entrance to the street leading to the Sandy Hook Elementary School on December 15. Hide Caption 9 of 43 Photos: Photos: Newtown school shooting Newtown school shooting – Corinne McLaughlin, a student at the University of Hartford, bows her head during a candlelight vigil at Hartford, Connecticut's Bushnell Park on Friday, December 14, honoring the students and teachers who died at Sandy Hook Elementary School in nearby Newtown earlier in the day. Hide Caption 10 of 43 Photos: Photos: Newtown school shooting Newtown school shooting – Distraught people leave the fire station after hearing news of their loved ones from officials on Friday. Hide Caption 11 of 43 Photos: Photos: Newtown school shooting Newtown school shooting – Emergency workers stand in front of the Sandy Hook Elementary School in Newtown. Hide Caption 12 of 43 Photos: Photos: Newtown school shooting Newtown school shooting – A child and her mother leave a staging area outside Sandy Hook Elementary School in Newtown, Connecticut, on December 14. Hide Caption 13 of 43 Photos: Photos: Newtown school shooting Newtown school shooting – Members of the media converge on December 14 in front of an apartment at 1313 Grand Street in Hoboken, New Jersey. The apartment is believed to be connected to the Connecticut elementary school shooting. Hide Caption 14 of 43 Photos: Photos: Newtown school shooting Newtown school shooting – Faisal Ali, right, of Colorado Springs, Colorado, joins other people outside the White House on December 14 to participate in a candlelight vigil to remember the victims of the Sandy Hook Elementary School shooting. Hide Caption 15 of 43 Photos: Photos: Newtown school shooting Newtown school shooting – Connecticut State Police spokesman Lt. J. Paul Vance, center, briefs the media on the elementary school shootings during a press conference at Treadwell Memorial Park on December 14 in Newtown. Hide Caption 16 of 43 Photos: Photos: Newtown school shooting Newtown school shooting – People weep and embrace near Sandy Hook Elementary School on Friday, December 14. Hide Caption 17 of 43 Photos: Photos: Newtown school shooting Newtown school shooting – A woman leans on a man as she weeps near Sandy Hook Elementary School on December 14. Hide Caption 18 of 43 Photos: Photos: Newtown school shooting Newtown school shooting – President Barack Obama wipes a tear as he speaks about the shooting at Sandy Hook Elementary School during a press briefing at the White House on December 14. Hide Caption 19 of 43 Photos: Photos: Newtown school shooting Newtown school shooting – A woman weeps near the site of a shooting at Sandy Hook Elementary School on December 14. Hide Caption 20 of 43 Photos: Photos: Newtown school shooting Newtown school shooting – A woman weeps near Sandy Hook Elementary School on December 14. Hide Caption 21 of 43 Photos: Photos: Newtown school shooting Newtown school shooting – People comfort each other near Sandy Hook Elementary School on December 14. Hide Caption 22 of 43 Photos: Photos: Newtown school shooting Newtown school shooting – A man takes in the scene near Sandy Hook Elementary School on December 14. Hide Caption 23 of 43 Photos: Photos: Newtown school shooting Newtown school shooting – A young girl is given a blanket after being evacuated from Sandy Hook Elementary School on December 14. Hide Caption 24 of 43 Photos: Photos: Newtown school shooting Newtown school shooting – State police personnel lead children from the school. Hide Caption 25 of 43 Photos: Photos: Newtown school shooting Newtown school shooting – Children wait outside Sandy Hook Elementary School in Newtown, Connecticut, after the shooting. Hide Caption 26 of 43 Photos: Photos: Newtown school shooting Newtown school shooting – A boy weeps at Reed Intermediate School after getting news of the shooting at Sandy Hook Elementary School on December 14. Hide Caption 27 of 43 Photos: Photos: Newtown school shooting Newtown school shooting – FBI SWAT team members walk along Dickinson Drive near Sandy Hook Elementary School on December 14. Hide Caption 28 of 43 Photos: Photos: Newtown school shooting Newtown school shooting – An aerial view of Sandy Hook Elementary School in Newtown, Connecticut on December 14. Hide Caption 29 of 43 Photos: Photos: Newtown school shooting Newtown school shooting – Connecticut State Troopers arrive on the scene outside Sandy Hook Elementary School on December 14. Hide Caption 30 of 43 Photos: Photos: Newtown school shooting Newtown school shooting – A Connecticut State Police officer runs with a shotgun at the Sandy Hook Elementary School in Newtown on December 14. Hide Caption 31 of 43 Photos: Photos: Newtown school shooting Newtown school shooting – Police patrol the streets around Sandy Hook Elementary School on December 14. Hide Caption 32 of 43 Photos: Photos: Newtown school shooting Newtown school shooting – People try to deal with the shock of the attack outside Sandy Hook Elementary School on December 14. Hide Caption 33 of 43 Photos: Photos: Newtown school shooting Newtown school shooting – Connecticut State Police secure the scene of the shooting on December 14. Hide Caption 34 of 43 Photos: Photos: Newtown school shooting Newtown school shooting – People embrace outside Sandy Hook Elementary School on December 14. Hide Caption 35 of 43 Photos: Photos: Newtown school shooting Newtown school shooting – A man escorts his son away from Sandy Hook Elementary School on December 14. Hide Caption 36 of 43 Photos: Photos: Newtown school shooting Newtown school shooting – People take in the news outside Sandy Hook Elementary School on December 14. Hide Caption 37 of 43 Photos: Photos: Newtown school shooting Newtown school shooting – People line up to enter Newtown Methodist Church near the the scene of the shooting on December 14. Hide Caption 38 of 43 Photos: Photos: Newtown school shooting Newtown school shooting – A woman speaks with a Connecticut state trooper outside Sandy Hook Elementary School on December 14. Hide Caption 39 of 43 Photos: Photos: Newtown school shooting Newtown school shooting – The streets around Sandy Hook Elementary are packed with first responders and other vehicles. Hide Caption 40 of 43 Photos: Photos: Newtown school shooting Newtown school shooting – A view of the scene at Sandy Hook Elementary School after the shooting. Hide Caption 41 of 43 Photos: Photos: Newtown school shooting Newtown school shooting – A young boy is comforted outside Sandy Hook Elementary School on December 14. Hide Caption 42 of 43 Photos: Photos: Newtown school shooting Newtown school shooting – People embrace each other on December 14. Hide Caption 43 of 43

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?

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"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.

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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 shootings

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.

Photos: Reaction to Newtown school killings Photos: Reaction to Newtown school killings Reaction to Newtown school killings – Candles burn next to a lighted tree at a makeshift shrine in Newtown, Connecticut, commemorating the victims of the mass shooting at Sandy Hook Elementary School on December 14, 2012. Hide Caption 1 of 80 Photos: Reaction to Newtown school killings Reaction to Newtown school killings – Erica Simmons rings the campus bell at Agnes Scott College in Decatur, Georgia, during a nationwide commemoration December 21, marking a week since the Newtown, Connecticut, mass shooting. Church bells rang out across the country at 9:30 ET Friday to remember those who died in the gun rampage. Hide Caption 2 of 80 Photos: Reaction to Newtown school killings Reaction to Newtown school killings – A woman pauses at a streetside memorial during a moment of silence on December 21 in Newtown. Hide Caption 3 of 80 Photos: Reaction to Newtown school killings Reaction to Newtown school killings – Connecticut State Police block the road to Sandy Hook Elementary School during a moment of silence on December 21. A week ago, a gunman forced his way into the school and shot and killed 26 people, including 20 children. Hide Caption 4 of 80 Photos: Reaction to Newtown school killings Reaction to Newtown school killings – A woman bows her head in Newtown's Sandy Hook village on December 21. Hide Caption 5 of 80 Photos: Reaction to Newtown school killings Reaction to Newtown school killings – People observe a moment of silence for the school shooting victims at the Blue Colony Diner in Newtown on December 21. Hide Caption 6 of 80 Photos: Reaction to Newtown school killings Reaction to Newtown school killings – Children in Newtown, excluding Sandy Hook Elementary, return to classes on Tuesday, December 18, four days after the shooting at the elementary school. Hide Caption 7 of 80 Photos: Reaction to Newtown school killings Reaction to Newtown school killings – U.S. Rep. David Cicilline, D-Rhode Island, holds a news conference with the Brady Campaign to discuss gun violence. In attendance with the Brady Campaign were several survivors of gun violence and family members of victims of gun violence. Hide Caption 8 of 80 Photos: Reaction to Newtown school killings Reaction to Newtown school killings – Members of the human rights group Volunteers Against Crime and Corruption light candles showing the names of those killed during the shooting at Sandy Hook Elementary School, during a prayer vigil in front of the U.S. Embassy in Manila, Philippines on December 18. Hide Caption 9 of 80 Photos: Reaction to Newtown school killings Reaction to Newtown school killings – Children light candles to pay their respects to the victims of the Newtown, Connecticut, shooting at the main square in Tirana, Albania, on Monday, December 17. The deadly gun rampage at Sandy Hook Elementary School has provoked strong reactions from around the world. Hide Caption 10 of 80 Photos: Reaction to Newtown school killings Reaction to Newtown school killings – Protesters march on the National Rifle Association's Capitol Hill lobbyist offices in Washington on December 17. Hide Caption 11 of 80 Photos: Reaction to Newtown school killings Reaction to Newtown school killings – New York Mayor Michael Bloomberg speaks out for stronger gun control at a press conference at City Hall on December 17. Bloomberg, co-chair of Mayors Against Illegal Guns, was joined by victims and survivors of gun violence. Hide Caption 12 of 80 Photos: Reaction to Newtown school killings Reaction to Newtown school killings – Chris Foye, whose son Chris Owens was killed by a stray bullet in 2009, stands with other survivors and family members of gun violence at Bloomberg's press conference on December 17 in New York. Hide Caption 13 of 80 Photos: Reaction to Newtown school killings Reaction to Newtown school killings – People pay their respects on December 17 at a makeshift shrine in Newtown to the victims of Friday's elementary school shooting. Funerals began Monday in the Connecticut town. Hide Caption 14 of 80 Photos: Reaction to Newtown school killings Reaction to Newtown school killings – Traders on the floor of the New York Stock Exchange hold a moment of silence on December 17 in honor of the shooting victims. Hide Caption 15 of 80 Photos: Reaction to Newtown school killings Reaction to Newtown school killings – Young people mourn at Newtown High School before a memorial service attended by President Obama on Sunday, December 16. Hide Caption 16 of 80 Photos: Reaction to Newtown school killings Reaction to Newtown school killings – President Barack Obama waits to speak at an interfaith vigil for the shooting victims from Sandy Hook Elementary School December 16 at Newtown High School. Hide Caption 17 of 80 Photos: Reaction to Newtown school killings Reaction to Newtown school killings – Mourners comfort one another December 16 before U.S. President Barack Obama speaks at an interfaith vigil for the shooting victims. Hide Caption 18 of 80 Photos: Reaction to Newtown school killings Reaction to Newtown school killings – Two women embrace before the interfaith vigil at Newtown High School on Sunday evening. Hide Caption 19 of 80 Photos: Reaction to Newtown school killings Reaction to Newtown school killings – From left: Newtown residents Claire Swanson, Kate Suba, Jaden Albrecht, Simran Chand and New London, Connecticut, residents Rachel Pullen and her son, Landon DeCecco, hold candles at a memorial for victims on Sunday, December 16, in Newtown, Connecticut. Hide Caption 20 of 80 Photos: Reaction to Newtown school killings Reaction to Newtown school killings – A young boy walks past Christmas trees set up at a makeshift shrine to the shooting victims in Newtown, Connecticut, on December 16. Hide Caption 21 of 80 Photos: Reaction to Newtown school killings Reaction to Newtown school killings – Ty Diaz is kissed by his mother, Yvette, at a memorial down the street from Sandy Hook on December 16. Hide Caption 22 of 80 Photos: Reaction to Newtown school killings Reaction to Newtown school killings – Teddy bears, flowers and candles in memory of those killed are left at a memorial down the street from the school on December 16. Hide Caption 23 of 80 Photos: Reaction to Newtown school killings Reaction to Newtown school killings – Two teenagers embrace at a makeshift shrine to the victims in Newtown on December 16. Hide Caption 24 of 80 Photos: Reaction to Newtown school killings Reaction to Newtown school killings – Nuns pay their respects at a makeshift shrine to the victims on December 16. Hide Caption 25 of 80 Photos: Reaction to Newtown school killings Reaction to Newtown school killings – Members of Sisters of Christian Charity go to lay flowers in front of the Sandy Hook Elementary School on December 16 in Newtown. Hide Caption 26 of 80 Photos: Reaction to Newtown school killings Reaction to Newtown school killings – A woman receives a hug as she leaves morning service December 16 at Trinity Church in Newtown near the elementary school. Hide Caption 27 of 80 Photos: Reaction to Newtown school killings Reaction to Newtown school killings – Parishioners pay their respects to the victims of the elementary school shooting while attending Mass at St. Rose of Lima Roman Catholic Church in Newtown on December 16. Hide Caption 28 of 80 Photos: Reaction to Newtown school killings Reaction to Newtown school killings – Police officers honor the victims of the school shooting at the St. Rose of Lima Roman Catholic Church on December 16 in Newtown. Hide Caption 29 of 80 Photos: Reaction to Newtown school killings Reaction to Newtown school killings – The Jacksonville Jaguars have a moment of silence in honor of the Sandy Hook Elementary School shooting victims before their game against the Miami Dolphins on December 16. Hide Caption 30 of 80 Photos: Reaction to Newtown school killings Reaction to Newtown school killings – A parishioner kneels in front of a makeshift memorial at St. Rose of Lima Roman Catholic Church in Newtown on December 16. Hide Caption 31 of 80 Photos: Reaction to Newtown school killings Reaction to Newtown school killings – A police officer removes flowers from a busy intersection on December 16 in Newtown. Police said they were afraid the memorial, left for the victims of the Sandy Hook Elementary School shooting, would cause a traffic hazard. Hide Caption 32 of 80 Photos: Reaction to Newtown school killings Reaction to Newtown school killings – A woman hugs her daughter on the steps of Trinity Church on December 16 in Newtown. Hide Caption 33 of 80 Photos: Reaction to Newtown school killings Reaction to Newtown school killings – Angel wood cutouts for each of the 27 victims are set up on hillside in Newtown on December 16. Hide Caption 34 of 80 Photos: Reaction to Newtown school killings Reaction to Newtown school killings – People in Bangalore, India, hold cards and photographs of the slain at a candlelight vigil outside a Catholic church on December 16. Hide Caption 35 of 80 Photos: Reaction to Newtown school killings Reaction to Newtown school killings – J.J. Watt of the Houston Texans shows his glove in remembrance of the victims before the start of a game against the Indianapolis Colts on December 16 in Houston. Hide Caption 36 of 80 Photos: Reaction to Newtown school killings Reaction to Newtown school killings – Donna Soto, right, mother of Victoria Soto, the first-grade teacher at Sandy Hook Elementary School who was shot and killed while protecting her students, hugs her daughter Karly while mourning their loss at a candlelight memorial at Stratford High School on Saturday, December 15, in Stratford, Connecticut. Hide Caption 37 of 80 Photos: Reaction to Newtown school killings Reaction to Newtown school killings – Firefighters kneel to pay their respects at a makeshift memorial near the school in Newtown on Saturday. Hide Caption 38 of 80 Photos: Reaction to Newtown school killings Reaction to Newtown school killings – A child lights a candle at a memorial filled with flowers, stuffed toys and candles outside of Saint Rose of Lima Church near Sandy Hook Elementary School in Newtown, Connecticut on Saturday. Hide Caption 39 of 80 Photos: Reaction to Newtown school killings Reaction to Newtown school killings – Candles light up a memorial outside of Saint Rose of Lima Church in Newtown. Hide Caption 40 of 80 Photos: Reaction to Newtown school killings Reaction to Newtown school killings – Lucas, Kelly and Michael DaSilva pray and embrace at a makeshift memorial near the school in Newtown. Hide Caption 41 of 80 Photos: Reaction to Newtown school killings Reaction to Newtown school killings – People are overcome with emotion Saturday at a makeshift memorial near Sandy Hook Elementary School in Newtown. Hide Caption 42 of 80 Photos: Reaction to Newtown school killings Reaction to Newtown school killings – Residents arrive Saturday to pay tribute to the victims of an elementary school shooting in Newtown, Connecticut. Hide Caption 43 of 80 Photos: Reaction to Newtown school killings Reaction to Newtown school killings – A couple carry balloons to place at a curbside shrine to in Newtown on Saturday. Hide Caption 44 of 80 Photos: Reaction to Newtown school killings Reaction to Newtown school killings – A mother and daughter attend a prayer service at St. John's Episcopal Church in Newtown on Saturday. Hide Caption 45 of 80 Photos: Reaction to Newtown school killings Reaction to Newtown school killings – Flowers and signs of sympathy adorn the street leading to Sandy Hook Elementary School. Hide Caption 46 of 80 Photos: Reaction to Newtown school killings Reaction to Newtown school killings – Claudia Urbiana and daughter Jocelyne Cardenas, left, hug outside of the entrance to the Sandy Hook school. Hide Caption 47 of 80 Photos: Reaction to Newtown school killings Reaction to Newtown school killings – A makeshift memorial with flowers, stuffed toys and candles sit outside Saint Rose of Lima Roman Catholic Church in Newtown, Connecticut, on Saturday. Hide Caption 48 of 80 Photos: Reaction to Newtown school killings Reaction to Newtown school killings – A man bows his head as he stands at a makeshift memorial, outside Saint Rose of Lima Roman Catholic Church in Newtown on Saturday, December 15. Hide Caption 49 of 80 Photos: Reaction to Newtown school killings Reaction to Newtown school killings – New Jersey resident Steve Wruble, who was moved to drive out to Connecticut to support local residents, grieves for victims at the entrance to Sandy Hook village in Newtown on Saturday. Hide Caption 50 of 80 Photos: Reaction to Newtown school killings Reaction to Newtown school killings – People attend a prayer service in Newtown on Saturday to reflect. Hide Caption 51 of 80 Photos: Reaction to Newtown school killings Reaction to Newtown school killings – A mother hugs her children after paying tribute to the victims in Newtown on Saturday, December 15. Hide Caption 52 of 80 Photos: Reaction to Newtown school killings Reaction to Newtown school killings – Newtown High School student Trevor Lopez stands outside of a church where residents have come to pray and reflect on Saturday Hide Caption 53 of 80 Photos: Reaction to Newtown school killings Reaction to Newtown school killings – Andrea Jaeger places flowers and a candle at a makeshift memorial outside a firehouse near Sandy Hook Elementary School on Saturday. Hide Caption 54 of 80 Photos: Reaction to Newtown school killings Reaction to Newtown school killings – The U.S. flag flies at half-staff above the White House on Saturday. Hide Caption 55 of 80 Photos: Reaction to Newtown school killings Reaction to Newtown school killings – Ken Kowalsky and his daughter Rebecca, 13, embrace while standing at the end of the road leading to Sandy Hook Elementary School on Saturday. Hide Caption 56 of 80 Photos: Reaction to Newtown school killings Reaction to Newtown school killings – A woman puts a flower near crosses planted by Rio de Paz (Rio of Peace), in memory of the victims of the Sandy Hook Elementary school shooting on Copacabana beach in Rio de Janeiro on Saturday. Hide Caption 57 of 80 Photos: Reaction to Newtown school killings Reaction to Newtown school killings – A woman sits during a service at the Asylum Hill Congregational Church in Hartford, Connecticut on Friday, December 14. Hide Caption 58 of 80 Photos: Reaction to Newtown school killings Reaction to Newtown school killings – Fans at the NBA game between the Utah Jazz and the Phoenix Suns participate in a moment of silence for the victims of the Newtown shooting on Friday in Phoenix. Hide Caption 59 of 80 Photos: Reaction to Newtown school killings Reaction to Newtown school killings – An overflow crowd listens to a church service held at the St. Rose of Lima Catholic Church in Newtown on December 14. Hide Caption 60 of 80 Photos: Reaction to Newtown school killings Reaction to Newtown school killings – People gather for a prayer vigil at St. Rose Church on Friday. Hide Caption 61 of 80 Photos: Reaction to Newtown school killings Reaction to Newtown school killings – People gather for a vigil outside the White House in Washington following the Connecticut elementary school shooting on Friday. Hide Caption 62 of 80 Photos: Reaction to Newtown school killings Reaction to Newtown school killings – Candles burn as people gather for a vigil outside the White House. Hide Caption 63 of 80 Photos: Reaction to Newtown school killings Reaction to Newtown school killings – Hartford, Connecticut, Mayor Padro Segarra speaks emotionally about the students and teachers who died earlier in the day at Sandy Hook Elementary School in nearby Newtown at a candlelight vigil at Bushnell Park in Hartford on Friday. Hide Caption 64 of 80 Photos: Reaction to Newtown school killings Reaction to Newtown school killings – Cynthia Alvarez is comforted by her mother, Lilia, as people gather for a prayer vigil at St. Rose of Lima Roman Catholic Church in Newtown. Hide Caption 65 of 80 Photos: Reaction to Newtown school killings Reaction to Newtown school killings – People gather for a prayer vigil at St. Rose Church in Newtown. Hide Caption 66 of 80 Photos: Reaction to Newtown school killings Reaction to Newtown school killings – People gather for a prayer vigil at St. Rose Church. Hide Caption 67 of 80 Photos: Reaction to Newtown school killings Reaction to Newtown school killings – People gather inside the St. Rose Church to remember the shooting victims on Friday Hide Caption 68 of 80 Photos: Reaction to Newtown school killings Reaction to Newtown school killings – A woman bows her head during a vigil for the shooting victims at St. Rose Church. Hide Caption 69 of 80 Photos: Reaction to Newtown school killings Reaction to Newtown school killings – People gather in the St. Rose Church for a memorial service Friday. Hide Caption 70 of 80 Photos: Reaction to Newtown school killings Reaction to Newtown school killings – Women comfort each other during the vigil at St. Rose Church. Hide Caption 71 of 80 Photos: Reaction to Newtown school killings Reaction to Newtown school killings – A woman looks on during the vigil at St. Rose Church. Hide Caption 72 of 80 Photos: Reaction to Newtown school killings Reaction to Newtown school killings – People hug outside of the Newtown United Methodist Church on Friday, near the site of the shootings at the Sandy Hook school. Hide Caption 73 of 80 Photos: Reaction to Newtown school killings Reaction to Newtown school killings – A flag at the U.S. Capitol flies at half-staff after President Barack Obama ordered the action while speaking from the White House. Obama called for "meaningful action" in the wake of the school shooting. Hide Caption 74 of 80 Photos: Reaction to Newtown school killings Reaction to Newtown school killings – Julie Henson of San Francisco joins other people outside the White House to participate in a candlelight vigil on Friday. Hide Caption 75 of 80 Photos: Reaction to Newtown school killings Reaction to Newtown school killings – Obama wipes tears as he makes a statement in response to the shooting on Friday. Hide Caption 76 of 80 Photos: Reaction to Newtown school killings Reaction to Newtown school killings – Washington resident Rachel Perrone, left, and her 5-year-old son, Joe, center, join others outside the White House in a candlelight vigil. Hide Caption 77 of 80 Photos: Reaction to Newtown school killings Reaction to Newtown school killings – Faisal Ali, right, of Colorado Springs, Colorado, joins the vigil outside the White House. Hide Caption 78 of 80 Photos: Reaction to Newtown school killings Reaction to Newtown school killings – People gather outside the White House to participate in a candlelight vigil. Hide Caption 79 of 80 Photos: Reaction to Newtown school killings Reaction to Newtown school killings – Supporters of gun control hold a candlelight vigil for victims of the shooting outside the White House. Hide Caption 80 of 80

Photos: Photos: Newtown grieves Photos: Photos: Newtown grieves Newtown grieves – Ken Henggeler started this memorial to the victims of the shooting at the intersection of Main Street and Sugar Street in Newtown, Connecticut. Hide Caption 1 of 6 Photos: Photos: Newtown grieves Newtown grieves – People pay their respects on Monday, December 17, at a memorial to the victims of an elementary school shooting in Newtown, Connecticut. Hide Caption 2 of 6 Photos: Photos: Newtown grieves Newtown grieves – Teddy bears show the names of some of the victims at a makeshift memorial in Newtown, Connecticut, on December 17. Hide Caption 3 of 6 Photos: Photos: Newtown grieves Newtown grieves – Newtown resident Palmer Chiaepetta walks with his sons Jonathan and Jackson as the American flag flies at half-staff in Newtown, Connecticut, on Sunday, December 16, two days after the mass shooting at Sandy Hook Elementary School. Hide Caption 4 of 6 Photos: Photos: Newtown grieves Newtown grieves – Names of victims are displayed on a flag in the business area of Newtown, Connecticut, on December 16. Hide Caption 5 of 6 Photos: Photos: Newtown grieves Newtown grieves – Two people embrace near a makeshift memorial in Newtown, Connecticut, on Saturday, December 15. Hide Caption 6 of 6

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.

Stepping in

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."