VIRGINIA Tech gunman Seung-Hui Cho fired more than 170 rounds in nine minutes, police revealed last night.
The maniac then killed himself with a bullet to his head in a classroom surrounded by victims.
But despite searching emails and phone records, detectives admitted they have not yet found a motive for the worst gun massacre in US history.
State Police Superintendent Col. W. Steven Flaherty said: "We talk about possible motives and theories but we don't have any evidence to suggest anything."
The investigation is expected to last months. Investigators have compiled 500 pieces of evidence from Norris Hall where Cho killed 30.
He had earlier killed Emily Hilscher, 18, and Ryan Clark, 22, in a dormitory across campus.