But they said investigators still don't know why Cho launched the bloody attack on his fellow students at Virginia Tech.
"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 searched computer files, cell phone records and e-mails, and had compiled 500 pieces of evidence from Norris Hall alone.
Yet so far, nothing indicated a motive or close link between the 23-year-old loner and his victims, Flaherty said.
Two hours before Cho chained shut three public entrances to Norris Hall and started his rampage there, he gunned down his first two victims in a dormitory across campus. In between, a package was mailed to NBC containing Cho's videotaped tirade and written manifesto about rich "brats" and their "hedonistic needs."
Flaherty, who is overseeing the investigative team looking at the shootings, said police have been unable to answer one of the case's most vexing questions: Why the spree began at the West Ambler Johnston dormitory, and why 18-year-old freshman Emily Hilscher was the first victim.
Witnesses place Cho outside West Ambler Johnston shortly before 7:15 a.m., when he fired the two shots that killed Hilscher and 22-year-old senior Ryan Clark, a resident assistant at the dorm, Flaherty said.
It is not known how Cho got in.
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.
"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."
Virginia Tech police chief Wendell Flinchum said Cho had a class this semester in Norris Hall, although it was not scheduled to meet on April 16, the day of the rampage.
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 Clark. 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.