Cho Seung Hui had an imaginary girlfriend who called him “Spanky”. He wrote plays full of death and paedophilia. He was so silent and menacing that frightened classmates stopped attending seminars. He took pictures under desks with his mobile phone aimed up female students’ skirts.

This was the deeply disturbing picture that emerged yesterday of the strange and silent world of the gunman who massacred 32 people at Virginia Tech on Monday.

His flatmates and teachers said that as long ago as 2005 they had grave concerns that he might kill himself or harm others; police said that he was known to them.

As it was disclosed that in the weeks before his shooting spree Cho was taking prescription medicine for depression, his former roommates spoke for the first time about his weird behaviour. One, who shared a bedroom with Cho in 2006, revealed that in an entire year the South Korean — he was raised in the United States and spoke perfect English — barely uttered a single word.

“He was just like a shadow,” John, the roommate said. He added that Cho, 23, slept with the light on and in the whole year he did not receive one visitor. He hung no posters or pictures on his wall and ate alone every night.

The two students who lived with Cho last year said that they informed university police after Cho stalked three female students, incidents that always began with incessant, anonymous instant messaging signed “Question Mark”, the name Cho used even when signing into classes.

Cho got so upset after police contacted him that he sent a message to Andy, one of the roommates, declaring: “I might as well kill myself now.”

He spent hours in his room downloading music on his laptop and listened repeatedly to Nirvana, Led Zeppelin and — “over and over again” — the song Shine by Collective Soul. He refused all attempts to engage him in conversation.

However, on one occasion after he had been to a party with John and Andy and had drunk several beers, Cho confided that he had an imaginary girlfriend, a supermodel. “She called him ‘Spanky’ and he called her ‘Jelly’,” Andy said. John said that after Cho started menacing female students, “I stopped telling friends to come by my rooms, especially girls.”

The first student who alerted police about Cho received a visit from him in November 2005. Cho told his roommates that he had gone to look into her eyes, and what he saw was “promiscuity”.

The English student wrote poems, a novel and two plays so disturbing that many of his classmates stopped attending classes. In one play, Mr Brownstone, three teenage characters talk of their hatred for Mr Brownstone, their maths teacher. “I wanna watch him bleed the way he makes us kids bleed,” says Jane, one of the characters. In the other play, Richard McBeef, John, a 13-year-old boy, accuses his stepfather of molesting him and killing his father. He tries to choke his stepfather with a “half-eaten banana cereal bar”. In another scene he rants: “I hate him. Must kill Dick. Dick must die.” It ends with the stepfather beating the boy to death.

Cho’s first victim on Monday was Emily Hilscher, 19, whom he shot in her dormitory at 7.15am. Heather Haugh, her roommate, said her friend did not know Cho. “I’ve never seen him,” she said. “I don’t know his name. Emily didn’t know him, as far as I know.”

Stephanie Derry was in a playwriting class with Cho. She says his plays were “really morbid and grotesque”. She added: “Cho was really, quiet. We always joked we were just waiting for him to do something. He even wrote one play about students being stalked by a teacher.”

Ian MacFarlane, a former classmate, said that reading Cho’s plays was like “something out of a nightmare”. He added: “When I first heard about the shootings my first thought was about my friends and my second was, ‘I bet it was Seung Cho.’ He was a loner, obsessed with violence.”

Cho’s current roommate, Karan Grewal, 21, saw Cho at 5.30am on Sunday. As usual, Cho was totally uncommunicative. Grewal said he noticed a handheld power drill and screwdriver in his room, which police believe Cho used to shave off the serial numbers on his two weapons, the Glock and a Walther P22 pistol.

Mr Grewal said that in recent weeks Cho had started going to the gym to lift weights and had trimmed his hair into a military buzz cut.

Cho moved to America in 1992, aged 8, with his parents, employees at a dry-cleaners in Centerville, a Washington suburb. Neighbours say they were devoted parents. Cho’s sister recently graduated from Princeton and now works for the State Department in Iraq.

Mr Brownstone

In Cho Seung Hui’s play, three high-school friends, John, Jane and Joe, all 17, are in a casino when they see their maths teacher

John: Do I smell evil around here?

Jane: Not just evil but old too

Joe: Old is bad enough. You mix evil with old, and rotting turd-hell on earth

Mr Brownstone (lowering his voice): You f***ing little kids. Don’t you publicly humiliate me! You know what I can do to you at school on Monday?

John: I feel a satantic presence around me. Do you guys feel it?

Times Recommends

• Should Knox’s trial even have reached the courtroom?
• 'Was it real?' The US bailout nightmare
• Iranian opposition plays the long game