By Emma Ketteley
BBC This World
What drove a shy, reserved Korean-American student to commit the worst mass shooting in US history?
Cho lived in the US with his family from the age of eight
On the morning of 16 April 2007, Cho Seung Hui shot dead 32 people on the campus of Virginia Tech university before turning his gun on himself.
Cho was born in South Korea and lived in Seoul until he was eight years old.
Kim Yang-Soon, the boy's great-aunt, remembers him as quiet and withdrawn.
"We didn't know that he was abnormal, we just thought he was docile."
Cho's grandfather, King Hyang-Sik, also saw nothing wrong. He describes the boy as "very quiet, but very smart".
But when his family migrated to Centreville in Virginia, Cho's teachers at Westfield High were so concerned about his shyness that they sent him to counselling.
"My memories are of him being alone, being isolated and being pretty miserable," says Theresa Fayne, who was in the same class as Cho. "He would communicate with a head nod."
This silence was to set Cho apart from his peers throughout his life.
His supervisory counsellor, Dr James Griffith, treated him throughout his teens and remembers how awkward Cho was.
"I recall the pain of his shyness. He would break out in a sweat and freeze with palpable pain at having to respond."
Cho's withdrawn behaviour remained with him when he arrived at Virginia Tech.
"He really went out of his way to make sure that no-one could interact with him. He would keep his head down and walk straight to his seat. He was the most anti-social person I have ever met," says classmate Josh Sweeney.
"In one class he refused to give his name and introduced himself as Question Mark," he added.
Cho's room-mate at Virginia Tech, Joseph Aust, said that he heard him speak "less than a handful" of times in the eight months that they shared the same space.
But despite the video diatribe sent to NBC in which Cho claims to have been persecuted, there is no evidence to show that he suffered any form of victimisation, either at school or Virginia Tech.
"I have no recollection of ever seeing anyone attack him either verbally or physically," says Joseph Aust.
But there was more to it than shyness.
Cho sent the video to NBC before carrying out the second shootings
At the age of 13, Cho was diagnosed by psychiatrist Dr James Griffith as having selective mutism and major depression.
Selective mutism is a condition whose sufferers can speak, but they have such severe social anxiety disorders that they withdraw in certain situations.
"Children with selective mutism have problems socially and emotionally. If left untreated this completely affects their self-worth," says Dr Elisa Shipon Blum, an authority on the subject.
"With many sufferers, as they get older, the social anxiety starts to go away, but their conditioned and reinforced mute behaviour persists."
Dr Griffith believes this is what happened with Cho.
"By the time he came to me, aged 13, it's difficult. This is not only an anxiety disorder; a whole lifestyle has been organised around it, and it is hard to change."
But selective mutism is not enough to explain what went wrong with Cho - and there is no link between violence and selective mutism.
While he was at Virginia Tech, Cho made a decision that seems bizarre for someone who found communicating so difficult. He switched majors from Business Information Technology to English.
His literary efforts came to the attention of his teachers, but not for the right reasons.
After one teacher was disturbed - by his refusal to participate verbally as well as the content of his work - Cho was taken under the wing of Lucinda Roy, then Head of English at Virginia Tech. She gave him private lessons.
Dr Roy recalls thinking that Cho could control his silence, and he actively chose to harness it as a tool to threaten.
"He would speak, and he could speak very eloquently if he wanted to," she recalls.
"He could also be very silent when he wanted to, and he seemed to use that silence in much the same way as he used his sunglasses and his hat - both as a way of hiding behind this persona he'd built, but also as a way of intimidation, of making sure that you knew that there were things about him you couldn't know."
Desire for revenge
Cho seemed to find refuge in fiction, and wrote many stories.
One play, titled Mr Brownstone, focuses on a group of kids winning at a casino. Their maths teacher tells the authorities the kids are under-age, and pockets their winnings. The teenagers threaten to kill him.
Dr Roger Depue, former head of the Behavioral Science Unit at the FBI, has analysed his writings.
He sees this drama as "a pretty good blueprint" of what was going on in Cho's mind.
"These are the injustices he is up against, and even if he wins, someone takes it away. There's a great desire for vengeance," Dr Depue suggests.
"The more specific a fantasy, the more dangerous it is. There are destructive fantasies in all of the writings - wanting to destroy others, and self-destruction."
But according to Lucinda Roy, Cho was no great writer and received low grades.
Child psychiatrist Bela Sood, from Virginia Commonwealth University, is one of the few to have interviewed Cho's parents since the massacre. She says he came from a caring and concerned family.
She believes that the approach of graduation may have triggered something.
"He knew he could not succeed, so he said, 'OK, how can I leave a mark in a way which no-one can take away from me? How do you do it in a way which will grab the world's attention?'
"He was moving away from the role of victim and donning this persona where aggression towards himself and aggression towards the world come together for this final act of revenge."
Tragically, Cho's act of revenge meant the murder of 32 innocent men and women.
This World: Massacre at Virginia Tech will be broadcast on Tuesday 8 April 2008 at 2100 BST on BBC Two. The programme is made by ORTV