While much remains unknown about the Virginia Tech University shooting, there are some common threads that run through the shooters that commit them.

The gunman responsible for the shooting rampage at Virginia Tech has been identified as Cho Seung-Hui, a South Korean male enrolled at the school, who was living on campus.

The incident, in which 33 people including the gunman died, is the deadliest mass shooting in U.S. history.

What has emerged since Monday's tragedy is that Cho was a loner who liked violent video games in his high school years. He has been accused of stalking women and setting a fire in his dorm room.

His creative writings were so disturbing (one play he wrote had a 13-year-old boy dying of a blow from the step-father he had accused of pedophilia) that the university's counselling department was informed. Some of his classmates actually wondered if Cho could be a potential school shooter some day. He was even temporarily detained over mental health concerns.

One police officer who saw a typed, eight-page note left behind by Cho said it was an angry, rambling, incoherent screed against rich kids and religion.

"'You caused me to do this'," the officer quoted the note as saying.

Cho indicated in his letter that the end was near and that there was a deed to be done, the officer said.

On Wednesday, NBC News reported that Cho had sent them materials on Monday after his first attack. In it, he referred to Dylan Klebold and Eric Harris, the two shooters of the 1999 Columbine High School incident, as "martyrs."

The Columbine question

The timing of Monday's event must be considered as coincidental to the eighth anniversary of the deadly April 20, 1999 rampage at Columbine High School in Littleton, Colo. that left 13 people and the two shooters dead.

Still, the Columbine massacre has clearly influenced Cho and other school shooters.

In 1999, just days after the Colorado incident, an angry young man who had dropped out of W.R. Myers high school in Taber, Alta. returned with a firearm and gunned down a former classmate.

The shooter, described as unpopular and the victim of bullying, was wearing a blue trench coat at the time. His family would later say his actions had been prompted by watching coverage of the Columbine killings.

At his trial in 2000, it came out that the teen had written: "I did this because, well, I'm not sure ... Ever since the shooting at Columbine ... all I've been able to think about is doing this."

Gunman Kimveer Gill -- who killed one person and injured 19 at Montreal's Dawson College in September 2006 -- left behind a note that praised Harris and Klebold.

Gill, a 25-year-old who washed out of the military and had an online presence at the goth website Vampirefreaks.com, enjoyed playing a video game based on the Columbine shooting.

In Calgary, a 14-year-old boy was arrested April 2. His parents tipped off the police that their son was threatening to kill teachers on the eighth anniversary of Columbine.

There were school and university shootings before Columbine, though they became more frequent in the 1970s and onward. However, the prominence of the Columbine shooting in the news media and popular culture created a whole new category for mass murder -- school shootings.

And once such a model becomes established, it becomes easier for like-minded people to follow it as they use the event to feed their fantasy mechanisms, Jordan Peterson, a clinical psychologist and University of Toronto professor, told CTV.ca.

Who does this?

Peterson doesn't paint a pretty picture of such killers, calling them self-pitying and narcissistic.

"Someone like this is usually profoundly alienated -- and they're arrogant, resentful and obsessive. They're unwilling to take responsibility for their own state of mind. They fantasize about revenge incessantly," he said.

That was clear in the Cho case, but also in Columbine, he said.

While everyone suffers serious hurt, loss or rejection at some point in their life, most people find a way to talk through it over otherwise get over it, he said.

"Someone like this starts to think about revenge and continues to think about it constantly ... building up a fantasy of revenge in their mind."

The Chos of this world are "lost in a world of pathological fantasy" and spiral down in a self-reinforcing loop, he said. "Once you start thinking that way, things can get out of control real quick."

This is understandable, "it's just a terrible thing to understand," Peterson said.

He noted some of the writings of Columbine killer Eric Harris, who was chillingly carefree about his final hours.

"He saw himself as a superhuman judge of existence itself. That may seem extreme, but this is extreme behaviour," he said.

An example of those writings: "'About 26.5 hours from now, the judgment will begin. Difficult, nerve-wracking but fun. What's life without a little bit of death? It's interesting, when I'm in my human form, to contemplate the fact that I won't be here in two days.'"

Peterson said those who carry out these acts are committing "conscious acts of revenge against the conditions of existence."

He thought the English poet John Milton, writing in his epic 1667 work "Paradise Lost," summed up the mindset of these individuals best:

"'The more I see pleasures around me, so much more I feel torment within me as from the hateful siege of contraries. All good to me becomes bane. And in heaven, much worse would be my state. ... Only in destroying do I find ease to my relentless thoughts.'"