Police provided new details about the case at a news conference on the Virginia Tech campus, but they said investigators still don't know what set the gunman off.

"We talk about possible motives and theories and whatnot, but we don't have any evidence to suggest anything," said State Police Superintendent Col. W. Steven Flaherty.

He said investigators had compiled 500 piece of evidence from Norris Hall alone.

Seung-Hui Cho chained shut three public entrances to Norris Hall before starting his rampage through the classrooms where he killed 30 students and teachers, police said. Two hours earlier, he had gunned down his first victims in a dormitory across campus.

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

Authorities found no close links between the 23-year-old loner and his victims, Flaherty said.

"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," Flaherty said. "It's frustrating because it's so personal, because we see the families and see the communities suffering, and we see they want answers."

Flaherty, who is overseeing the investigative team looking at the shootings, said police also have been unable to answer one of the case's most vexing questions: Why the spree began at the West Ambler Johnston dorm, and why 18-year-old freshman Emily Hilscher was the first victim.

Police searched Hilscher's e-mails and phone records looking for a link. While Flaherty would not discuss exactly what police found, he said neither Cho's nor Hilscher's records have revealed a connection.

Flaherty cautioned that it could be months before the case is closed. The investigation will begin slowing down as authorities examine evidence, he said.

"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 told The Associated Press in an interview.

Flaherty said they found no link between Cho and 22-year-old senior Ryan Clark, who was also killed at the dorm where he was a resident assistant. Nor do investigators know why Cho, an English major, selected Norris Hall — a building that is home primarily to engineering offices.

Frustrating their 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," Flaherty said.