ST. LOUIS - One of the two armed seniors at Columbine High School in Colorado who murdered 12 students and a teacher had shown signs that he was a fledgling psychopath with antisocial traits - someone capable of becoming a coldblooded killer.
The gunman who murdered 32 people in the 2007 Virginia Tech massacre had been diagnosed with a severe anxiety disorder.
Adam Lanza, the 20-year-old shooter at Sandy Hook Elementary School in Newtown, Conn., was known to be socially awkward and had difficulty coping with daily life.
In recent decades, mental-health experts and law enforcement officials have attempted to use "threat assessment" principles to help identify people who may be a danger to the community.
But there is no simple formula to determine who might be the next shooter, experts say, and such attempts at profiling run the risk of misidentifying individuals who may only be suffering from depression or a behavioral disorder.
People who constantly spew threats of violence may never follow through on their words, and someone who seems sociable and mild-mannered can turn out to be the next mass murderer.
"We don't know everything that goes on in the brain and what sets these things off," said Dr. Annemarie Loth, a pediatric psychiatrist and professor at St. Louis University. "Behavior is still a choice. Our environment plays a role. Their biology plays a role."
However, she and other mental-health experts stressed that psychiatric care and medication can go a long way toward treating a person's illness.
In 2002, the U.S. Secret Service found in its investigation of 37 school shootings that while most all of these massacres are planned, there is no accurate or useful profile of students who engage in targeted school violence. Not much has changed to alter that conclusion, except that the list of horrific massacres has grown.
The Secret Service found that most of the shooters - whose ages ranged from 11 to 21 - came from two-parent families, socialized with mainstream students, had no history of violent or criminal behavior, and had never or rarely been in trouble at school.
Only a third of the attackers had received a mental-health evaluation, but most had exhibited a history of suicide attempts or thoughts. Many of the attackers had felt bullied, or persecuted.
"It's very hard to predict, and if you cast too big a net, then what do you do?" said John Eiler, president of the St. Louis Regional Psychiatric Stabilization Center, which has short-term beds for people in crisis. "What would we have done in Aurora (the site of a mass shooting in July inside a Colorado movie theater) or Connecticut? Would we have had enough information beforehand to say: This person needs to be institutionalized? Probably not."
After the Columbine massacre in 1999, the FBI created a "school shooter" assessment that it said would help teachers spot potential killers among students.
The study listed various traits, from comments and writings that show an obsession with violence to attitudes of intolerance and superiority as well as the development of negative role models such as Adolf Hitler or Satan.
But the FBI conceded that "there is no magic number of traits or constellation of traits which will determine what students may present a problem."
Since the Virginia Tech massacre, college administrators across the country have focused on identifying potentially dangerous students.
Many colleges have created "threat assessment teams" of administrators, counselors and campus police officers to track disruptive students and those with mental-health issues. The teams monitor and interview students of concern, attempt to gauge the credibility of specific threats and to intervene.
At the University of Missouri in Columbia, the "at-risk behaviors team" meets bimonthly in part to discuss individual students who are deemed to be a potential danger. Mizzou conducts screenings for depression and stress - and helps connect students with counselors.
"It's hard to measure what you prevent, but we feel like we have done pretty well," said Cathy Scroggs, vice chancellor for student affairs.
High schools and middle schools, too, are attempting to recognize the warning signs and to manage overly aggressive students.
Dr. Eric Rossen, director of professional development and standards at the National Association of School Psychologists in Bethesda, Md., said that threat assessment has "helped at least to some degree to identify students who are at heightened risk" and provides appropriate interventions.