At least 26 others were injured in the shootings, police said.
The FBI and the Bureau of Alcohol, Tobacco, Firearms and Explosives believe the gunman, described as a young Asian male, used two handguns in the shootings before taking his own life, sources tell CBS News. One official added that the gunman was "heavily armed and wearing a vest."
Investigators offered no motive for the attack but said they are trying to confirm if the gunman was looking for his girlfriend, CBS News reports. The gunman's name was not immediately released, and it was not known if he was a student.
"Today the university was struck with a tragedy that we consider of monumental proportions," said Virginia Tech president Charles Steger. "The university is shocked and indeed horrified."
Students complained that there were no public-address announcements or other warnings on campus after the first burst of gunfire. They said the first word they received from the university was an e-mail more than two hours into the rampage — around the time the gunman struck again.
Steger said authorities at first believed that the shooting at the dorm was a domestic dispute and that the gunman had fled the campus.
"We had no reason to suspect any other incident was going to occur," he said, adding, "We can only make decisions based on the information you had at the time," he said. "You don't have hours to reflect on it."
The shootings spread panic and confusion on campus. Witnesses reporting students jumping out the windows of a classroom building to escape the gunfire. SWAT team members with helmets, flak jackets and assault rifles swarmed over the campus. Students and faculty members carried out some of the wounded themselves, without waiting for ambulances to arrive.
The massacre took place at opposite sides of the 2,600-acre campus, beginning at about 7:15 a.m. at West Ambler Johnston, a coed dormitory that houses 895 people, and continuing at least two hours later at Norris Hall, an engineering and general classroom building about a half-mile away, authorities said.
Katie Couric will anchor The CBS Evening News from Blacksburg, Va., tonight at 6:30 p.m. and Harry Smith will co-anchor The Early Show from there Tuesday morning from 7 to 9 a.m. There will a one-hour special on the massacre on 48 Hours Tuesday night at 10 p.m.
Photo Essay: Virginia Tech Massacre
Virginia Tech Police Chief Wendell Flinchum said one male and one female student were killed in the dormitory shooting. The rest were killed at Norris Hall.
Officials are confident there was one gunman and that the shootings were not part of a larger plot, CBS News reports. FBI spokesman Richard Kolko in Washington said there was no immediate evidence to suggest it was a terrorist attack, "but all avenues will be explored."
Some students bitterly questioned why the gunman was able to strike a second time, two hours after the bloodshed began.
"What happened today, this was ridiculous," student Jason Piatt told CNN. He said the first warning from the university of a shooting on campus came in an e-mail about two hours after the first deadly burst of gunfire. "While they're sending out that e-mail, 22 more people got killed," Piatt said.
Students and Laura Wedin, a student programs manager at Virginia Tech, said the first notification they got of the shootings came in an e-mail at 9:26 a.m., more than two hours after the first shooting.
The e-mail had few details. It said: "A shooting incident occurred at West Amber Johnston earlier this morning. Police are on the scene and are investigating." The message warned students to be cautious and contact police about anything suspicious.
Student Maurice Hiller said he went to a 9 a.m. class two buildings away from the engineering building, and no warnings were coming over the outdoor public address system on campus at the time.
Everett Good, junior, said of the lack of warning: "I'm trying to figure that out. Someone's head is definitely going to roll over that."
"We were kept in the dark a lot about exactly what was going on," said Andrew Capers Thompson, a 22-year-old graduate student from Walhalla, S.C.
At least 26 people were being treated at three area hospitals for gunshot wounds and other injuries, authorities said. Their exact conditions were not disclosed, but at least one was sent to a trauma center and six were in surgery, authorities said.
Up until Monday, the in U.S. history was in Killeen, Texas, in 1991, when George Hennard plowed his pickup truck into a Luby's Cafeteria and shot 23 people to death, then himself.
The massacre Monday took place almost eight years to the day after the Columbine High bloodbath near Littleton, Colo. On April 20, 1999, two teenagers killed 12 fellow students and a teacher before taking their own lives.
Previously, the deadliest campus shooting in U.S. history was a rampage that took place in 1966 at the University of Texas at Austin, where Charles Whitman climbed the clock tower and opened fire with a rifle from the 28th-floor observation deck. He killed 16 people before he was shot to death by police.
Founded in 1872, Virginia Tech is nestled in the Blue Ridge Mountains of southwestern Virginia, about 160 miles west of Richmond. With more than 25,000 full-time students, it has the state's largest full-time student population. The school is best known for its engineering school and its powerhouse Hokies football team.
The rampage took place on a brisk spring day, with snow flurries swirling around the campus. The campus is centered around the Drill Field, a grassy field where military cadets — who now represent a fraction of the student body — once practiced. The dorm and the classroom building are on opposites sides of the Drill Field.
A gasp could be heard at a campus news conference when Virginia Tech Police Chief W.R. Flinchum said at least 20 people had been killed. Previously, only one person was thought to have been killed.
Investigators from the federal Bureau of Alcohol, Tobacco, Firearms and Explosives began marking and recovering the large number of shell casings and will trace the weapon used, authorities said.
After the shootings, all entrances to the campus were closed, and classes were canceled through Tuesday. The university set up a meeting place for families to reunite with their children. It also made counselors available and planned an assembly for Tuesday at the basketball arena.
After the shooting began, students were told to stay inside away from the windows.
Aimee Kanode, a freshman from Martinsville, said the shooting happened on the fourth floor of West Ambler Johnston dormitory, one floor above her room. Kanode's resident assistant knocked on her door about 8 a.m. to notify students to stay put.
Police said there had been bomb threats on campus over the past two weeks by authorities but said they have not determined a link to the shootings.
It was second time in less than a year that the campus was closed because of a shooting.
Last August, the opening day of classes was canceled and the campus closed when an off campus and fled to the Tech area. A sheriff's deputy involved in the manhunt was killed on a trail just off campus. The accused gunman, William Morva, faces capital murder charges.
President Bush said the mass shooting affects every student across the nation.
"Schools should be places of safety, sanctuary and learning," Bush said. "When that sanctuary is violated, the impact is felt in every American classroom in every American community."
Bush spoke with Gov. Timothy M. Kaine and Virginia Tech President Charles Steger.
"I told them that Laura and I and many across our nation are praying for the victims and all the members of university community that have been devastated by this terrible tragedy," Bush said in the Diplomatic Room of the White House.
In the House, which returned Monday from a two-week recess, Speaker Nancy Pelosi, D-Calif., interrupted the proceedings to lead a moment of silence in remembrance.
"As the Virginia Tech community struggles with the mourning and questioning that is certain to follow, the continued prayers from this Congress are with the students, their families, the faculty and the staff at Virginia Tech," Pelosi said.