By Kathryn Westcott
Cho Seung-hui was a deeply disturbed young man - as the shocking and horrifying video he left behind reveals.
But did he fit some kind of killer's profile? It appears that the 23-year-old's actions and intentions fell into a pattern of previous school and college shootings.
Cho was known to the police before the shooting
Robin Kowalski, a psychologist at Clemson University in South Carolina, has co-written a comprehensive study on school shootings. She told the BBC News website that Cho's actions bore strong similarities with other killings.
Like Eric Harris and Dylan Klebold - perpetrators of the Columbine massacre - and Kimveer Gill, who went on a killing spree at a Canadian school last year, the foundation for Cho's violent act was laid out long before it took place.
And, like those killers, he left behind rage-filled testaments - Harris, Klebold and Kimveer via the internet, Cho via his video.
Cho even refers to the Columbine perpetrators as "martyrs" in his video, which bore similarities to images on Kimveer's own web page. On it, Kimveer is shown brandishing a gun and a hunting knife. Cho's own video also shows him with guns and a knife.
"We distinguished five common factors in the shootings we studied," says Ms Kowalski. "The first is an acute rejection episode - such as a break up with a girlfriend - which usually takes place shortly before the killer acts."
It is not known at this stage whether Cho had experienced an acute rejection episode. But in his video, he makes it clear he believes there was an on-going history of rejection, another factor, according to Ms Kowalski.
Kimveer Gill said he wanted to die in a hail of gunfire
"Teasing, bullying or other kinds of rejection were common elements in school shootings," she says.
In his video, Cho says: "You have vandalised my heart, raped my soul and tortured my conscience. You thought it was one pathetic more life you were extinguishing. Thanks to you, I die like Jesus Christ to inspire generations of the weak and the defenceless people."
He also labelled fellow students "brats" and "snobs".
A fascination with guns and explosives is another factor. Cho, it appears, had acquired weaponry over a period of time. The video also shows him dressed in a hunting vest, black baseball cap and gloves brandishing guns at the camera.
Another factor is a pre-occupation with death. "They talk about it a lot and they think about it. Cho's plays appear to indicate a fascination with the subject," says Ms Kowalski.
In one of his plays, entitled Richard McBeef, the main character named John is alone in his room throwing darts at a target covered with a picture of his stepfather, the eponymous character. John says: "I hate him. Must kill Dick. Must kill Dick. Dick must die. Kill Dick."
Cho sent the video to NBC before carrying out the second shootings
Ms Kowalski also found that perpetrators had underlying psychological problems.
Cho had previously been accused of stalking two female students, and had been taken to a mental health facility in 2005. There were also concerns at the time that he was suicidal.
Steve Hinshaw, chair of the psychology department at the University of California
Berkeley, told AFP news agency, that while it was impossible to accurately diagnose Cho from the brief
set of video clips, his actions and words showed Cho could have been suffering from a severe case of grandiosity and possibly either bipolar depression or schizophrenia.
"He made a statement that he won't be put down and this must be shown in a self-destructive but self-promoting blaze of glory," he said.
Secret Service study
Ms Kowalski said that in many campus and school killings, the perpetrators planned their attacks some time in advance. Cho must have planned the attack more than a month ago, when he purchased his first gun. It appears that he began working on material for his video at least six days before the shootings:
"This guy was very methodical and very calculated," says Ms Kowalski. "He was very clear cut about what he wanted to do."
In 2002, the US Secret Service conducted a major study of 37 school shootings to learn the patterns of the school-aged assassins.
Most attacks, the report said, come from loners with some kind of grievance. It found that more than half had revenge as a motive.
The Columbine massacre was well-planned
It also found that such shooters do not snap - they plan and they usually tell a friend or a classmate before the attack.
It also found that in 75% of cases, at least one adult had raised concern about the attacker before the incident happened because of his behaviour.
Cho's creative writing was so disturbing that he was referred to the school's counselling service.
Dr Scott Poland, is a psychologist at Nova Southeastern University in Fort Lauderdale and an expert on school killings.
"The focus right now is on trying to figure out why Cho did what he did - but even in school shootings where the perpetrators have survived, we have often been left with many unanswered questions," he told the BBC News website.
According to the Secret Service, there is no accurate or useful "profile" of students who engaged in targeted school violence.
It says that profiling students who may have traits similar to those of school shooters is not an effective remedy. Experts say that the concern with profiling is that it can include a high percentage of students that have similar characteristics.
"It will take the right set of personality characteristics in addition to certain environmental triggers to produce such events. There are many students who are troubled and many who have a fascination with death, but it doesn't mean that they are going to kill people," says Ms Kowalski.
Such attacks have a tremendous and lasting effect on the place that was targeted. In the aftermath, questions are inevitably asked about whether the authorities should have known that the attack was being planned, and could have been prevented.
"This young man was known to be different and unique and one of the issues at college campuses is whether or not we can mandate any kind of counselling," says Professor Poland.
"Do we, for example, have the leverage to make a student who is troubled come to the counselling service?
"We need to make sure that students know that the counselling service always has trained psychologists that can help them."
He added that he would like to see the focus shifted on to the needs of the surviving students and teachers.
"My experience is that the school often underestimates the long-term effect of the crisis. It is not unusual for there to be a lot of depression and suicides in the weeks and months after the tragedy."