It's the kind of crime that defies comprehension: an angry man with a gun walks into a school and picks his victims at random.
He is no serial killer, carefully stalking his prey. He is no vengeful husband, destroying the family he built and lost.
He kills, seemingly, without rhyme or reason and then turns the gun on himself.
Few details have emerged about the man who killed at least 30 people at Virginia Tech University in the bloodiest school shooting in US history Monday.
Students described him as "Asian-looking," wearing a brown hiking-type shirt with a black vest over it, and standing around six feet tall.
They said he repeatedly fired on cowering students and staff in an attempt to kill as many people as possible. Police recovered two weapons and said doors to the building had been chained from the inside.
The most common profile for a mass murderer is the lone gunman: someone who is "isolated, reclusive and antisocial," said Alan Langlieb, director of workplace psychiatry at Johns Hopkins University.
When asked to explain how anyone could commit such a horrific crime, Langlieb said simply "there's no answer."
"It's not exactly clear what snaps," he said, explaining that human behavior is too complex to lend itself to straightforward answers.
"Some of it is premeditated, but a person could wake up that day and decide I'm going to create social havoc."
There are very few people who have enough disregard for human life to perpetrate such horrific acts of random violence and most of those will never carry out such an act, Langlieb said.
But for someone who is pathologically unstable, socially isolated, angry at the world and out to make a statement, something as simple as a news story could be enough to set them off.
And it is nearly impossible to figure out ahead of time when that will happen and who will be the one to snap.
"On the wrong day with the right weapon and the right confluence of events you can have a catastrophe," Langlieb said.
The US Secret Service has said "there is no accurate or useful 'profile' of students who engaged in targeted school violence."
It reached those conclusions after studying 37 incidents of targeted school violence involving 41 attackers in the United States between 1974 and June 2000.
Very few of the attackers were failing in school and some were even on the honor roll. Nearly two thirds had never been in trouble at school and most showed no marked change in academic performance, interest in school, friendship patterns or school disciplinary problems prior to the attack.
The largest group of shooters were considered to be "mainstream" students (41 percent) but about a third were characterized as "loners" and about a quarter were part of a "fringe" group.
Most attackers had no history of violence or criminal behavior, but about three quarters of the attackers felt "persecuted, bullied, threatened, attacked or injured by others prior to the attack."
They were also likely to have had trouble "coping with significant losses or personal failures" and 98 percent had "experienced or perceived some major loss prior to the attack."
The study found that these attacks are rarely "sudden, impulsive acts:" more than half of the shooters had been planning the attack for at least a month and more than two thirds of the attackers had told more than one person about the plan before they carried it out.
A study by the FBI found students engage in school violence often displayed a preoccupation with violence, depression, narcissism, alienation, lack of empathy, anger management problems, intolerance and a lack of trust in others.
Blaming society or violence on television is not the answer, Langlieb said, noting that people react very differently to the same situation and that antisocial personalities develop at an early age and seem to have been present throughout history and across cultures.
"Society can do more in a preventative way to get help and support for people who are in need but that's very different from saying living in a stressful environment makes people engage in road rage," he said.
"People have been angry through all time and we're now seeing it manifest in different ways. It's not so much what goes on in the environment as much as how people cope with stress."
