The threat of suspicious activity turned out to be unfounded, said Virginia State Police spokeswoman Corinne Geller said, and the building was reopened. But students were rattled.
"They were just screaming, 'Get off the sidewalks,'" said Terryn Wingler-Petty, a junior from Wisconsin. "They seemed very confused about what was going on. They were just trying to get people organized."
One officer was seen escorting a crying young woman out of Burruss Hall, telling her, "It's OK. It's OK."
The student who had killed 32 people in two university buildings on Monday had also killed himself, but he left behind a bomb threat, and the school was investigating whether he had any links to previous bomb threats on campus.
Cho Seung-Hui's roommates and professors on Wednesday described a troubled, very quite young man who rarely spoke to his roommates or made eye contact with them. His bizarre behavior became even less predictable in recent weeks, roommates Joseph Aust and Karan Grewal said.
Grewal said he had pulled an all-nighter on homework before the shootings and saw Cho at around 5 a.m.
"He didn't look me in the eye. Same old thing. I left him alone," He told CNN. He said when he saw Cho that morning and during the weekend, Cho didn't smile, didn't frown and didn't show any signs of anger. Grewal also said he never saw any weapons.
Several students and professors described Cho as a sullen loner. Authorities said he left a rambling note raging against women and rich kids. News reports said that Cho, a 23-year-old senior majoring in English, may have been taking medication for depression and that he was becoming increasingly erratic.
Professors and classmates were alarmed by his class writings _ pages filled with twisted, violence-drenched writing.
"It was not bad poetry. It was intimidating," poet Nikki Giovanni, one of his professors, told CNN Wednesday.
"I know we're talking about a youngster, but troubled youngsters get drunk and jump off buildings," she said. "There was something mean about this boy. It was the meanness _ I've taught troubled youngsters and crazy people _ it was the meanness that bothered me. It was a really mean streak."
Giovanni said her students were so unnerved by Cho's behavior, including taking pictures of them with his cell phone, that some stopped coming to class and she had security check on her room. She eventually had him taken out of her class, saying she would quit if he wasn't removed.
Lucinda Roy, a co-director of creative writing at Virginia Tech, said she tutored Cho after that.
"He was so distant and so lonely," she told ABC's "Good Morning America" Wednesday. "It was almost like talking to a hole, as though he wasn't there most of the time. He wore sunglasses and his hat very low so it was hard to see his face."
Roy also described using a code word with her assistant to call police if she ever felt threatened by Cho, but she said she never used it.
Cho's writing was so disturbing, though, he was referred to the university's counseling service, said Carolyn Rude, chairwoman of the university's English department.
In screenplays Cho wrote for a class last fall, characters throw hammers and attack with chainsaws, said a student who attended Virginia Tech last fall. In another, Cho concocted a tale of students who fantasize about stalking and killing a teacher who sexually molested them.
"When we read Cho's plays, it was like something out of a nightmare," former classmate Ian MacFarlane, now an AOL employee, wrote in a blog posted on an AOL Web site.
"The plays had really twisted, macabre violence that used weapons I wouldn't have even thought of."
He said he and other students "were talking to each other with serious worry about whether he could be a school shooter."
"We always joked we were just waiting for him to do something, waiting to hear about something he did," said another classmate, Stephanie Derry. "But when I got the call it was Cho who had done this, I started crying, bawling."
Despite the many warning signs that came to light in the bloody aftermath, police and university officials offered no clues as to exactly what set Cho off.
Cho _ who arrived in the United States as boy from South Korea in 1992 and was raised in suburban Washington, D.C., where his parents worked at a dry cleaners _ left a note that was found after the bloodbath.
A law enforcement official described it Tuesday as a typed, eight-page rant against rich kids and religion. The official spoke on condition of anonymity because he was not authorized to speak to the media.
"You caused me to do this," the official quoted the note as saying.
Cho indicated in his letter that the end was near and that there was a deed to be done, the official said. He also expressed disappointment in his own religion, and made several references to Christianity, the official said.
The official said the letter was either found in Cho's dorm room or in his backpack. The backpack was found in the hallway of the classroom building where the shootings happened, and contained several rounds of ammunition, the official said.
With classes canceled for the rest of the week, many students left town.
Tuesday night, thousands of Virginia Tech students, faculty and area residents poured into the center of campus to grieve together. Volunteers passed out thousands of candles in paper cups, donated from around the country. Then, as the flames flickered, speakers urged them to find solace in one another.
As silence spread across the grassy bowl of the drill field, a pair of trumpets began to play taps. A few in the crowd began to sing Amazing Grace.
Afterward, students, some weeping, others holding each other for support, gathered around makeshift memorials, filling banners and plywood boards with messages belying their pain.
"I think this is something that will take a while. It still hasn't hit a lot of people yet," said Amber McGee, a freshman from Wytheville, Va.
Monday's rampage consisted of two attacks, more than two hours apart _ first at a dormitory, where two people were killed, then inside a classroom building, where 31 people, including Cho, died. Two handguns _ a 9 mm and a .22-caliber _ were found in the classroom building.
According to court papers, police found a "bomb threat" note _ directed at engineering school buildings _ near the victims in the classroom building. In the past three weeks, Virginia Tech was hit with two other bomb threats. Investigators have not connected those earlier threats to Cho.
Cho graduated from Westfield High School in Chantilly, Va., in 2003. His family lived in an off-white, two-story townhouse in Centreville, Va.
At least one of those killed in the rampage, Reema Samaha, graduated from Westfield High in 2006. But there was no immediate word from authorities on whether Cho knew the young woman and singled her out.
"He was very quiet, always by himself," neighbor Abdul Shash said. Shash said Cho spent a lot of his free time playing basketball and would not respond if someone greeted him.
Some classmates said that on the first day of a British literature class last year, the 30 or so students went around and introduced themselves. When it was Cho's turn, he didn't speak.
On the sign-in sheet where everyone else had written their names, Cho had written a question mark. "Is your name, `Question mark?'" classmate Julie Poole recalled the professor asking. The young man offered little response.
Cho spent much of that class sitting in the back of the room, wearing a hat and seldom participating. In a small department, Cho distinguished himself for being anonymous. "He didn't reach out to anyone. He never talked," Poole said.
"We just really knew him as the question mark kid," Poole said.
One law enforcement official said Cho's backpack contained a receipt for a March purchase of a Glock 9 mm pistol. Cho held a green card, meaning he was a legal, permanent resident. That meant he was eligible to buy a handgun unless he had been convicted of a felony.
Roanoke Firearms owner John Markell said his shop sold the Glock and a box of practice ammo to Cho 36 days ago for $571.
"He was a nice, clean-cut college kid. We won't sell a gun if we have any idea at all that a purchase is suspicious," Markell said.
Investigators stopped short of saying Cho carried out both attacks. But State Police ballistics tests showed one gun was used in both.
And two law enforcement officials, speaking on condition of anonymity because the information had not been announced, said Cho's fingerprints were on both guns. Their serial numbers had been filed off.
Gov. Tim Kaine said he will appoint a panel at the university's request to review authorities' handling of the disaster. Parents and students bitterly complained that the university should have locked down the campus immediately after the first burst of gunfire and did not do enough to warn people.
Kaine warned against making snap judgments and said he had "nothing but loathing" for those who take the tragedy and "make it their political hobby horse to ride."
"I'm satisfied that the university did everything they felt they needed to do with the heat on the table," Kaine told CBS' "The Early Show" on Wednesday. "Nobody has this in the playbook, there's no manual on this."