The gunman who carried out the massacre at Virginia Tech university fired more than 170 shots during a rampage that lasted nine minutes, police said yesterday as they released new details about the tragedy.

Cho Seung Hui also had more ammunition left when he killed himself, in a classroom surrounded by many of his victims, with a single bullet to the head from one of the two handguns he used in the massacre.

In Norris Hall, the classroom building where Cho killed 30 of his 32 victims, he first chained shut its three public entrances before unleashing the rampage. Police said 25 people were injured in the massacre, some as they leapt from windows to escape the carnage.

Two hours earlier, at just after 7am, Cho killed his first victim, Emily Hilscher, 19, in her dormitory room at Ambler Johnston Hall.

Computer files, mobile phone records and e-mails have yielded no evidence about what triggered last week’s massacre, or whether he singled out any of his 32 victims.

Police said that they had found no evidence linking Cho to Miss Hilscher in any way, but witnesses saw him hanging around the main door of her dormitory building just before 7am.

Steve Flaherty, of the Virginia State Police, said Cho had practised firing the 9mm Glock and P22 Walther handguns at two local firing ranges in the weeks before the massacre. He said that investigators had compiled 500 pieces of evidence from Norris Hall, but still had no answers about what motivated Cho, 23, to carry out such a bloody killing spree.

“We talk about possible motives and theories and whatnot, but we don’t have any evidence to suggest anything.”

He added: “We certainly don’t have any one motive that we are pursuing at this particular time, or that we have been able to pull together and formulate. It’s frustrating because it’s so personal, because we see the families and see the communities suffering, and we see they want answers.” Between his first murders in Ambler Hall, and the rampage in Norris Hall, Cho took time out to walk to Blackburg’s post office and send a DVD and manuscript to NBC television in New York. In the video clips, he railed against wealthy students and “debauchery” on campus.

Police said that they were still not sure if Cho carried all the ammunition used in the attacks on himself, or if he had prepositioned some of it before entering Norris Hall last Monday morning.

Colonel Flaherty cautioned that it could be months before the case is closed. Classes resumed at Virginia Tech on Monday, but students who do not want to attend the last few weeks of term have been given permission to stay away.

“If we get to a point when we reach the end of this investigation, whenever it is, and we don’t have those answers that they need, it’s really difficult to sit down and say I just don’t know,” he said. Frustrating the effort is the fact that Cho revealed himself to so few people. Even family members have said they rarely heard him speak.

“I guess the thing that is most startling to me, I say startling, surprising, is a young man who’s 23 years old, that’s been here for a while, that seemed to not know anybody,” Colonel Flaherty said.

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