Police have named a student who shot dead at least 30 people at a US university on Monday as Cho Seung-hui, a 23-year-old South Korean.
Cho was studying English and had been living on the university campus. He killed himself after the rampage.
He is also thought to have killed two people earlier in the day. It was the worst gun massacre in US history.
Virginia Governor Tim Kaine on Tuesday pledged to immediately launch an independent review of the events.
Earlier in the day, US President George Bush told a university memorial service it was "a day of sadness for our entire nation".
Mr Bush, attending with his wife Laura, called for people to offer help to bereaved relatives, and said: "In this time of anguish, I hope you know that people all over this country are thinking about you and asking God to provide comfort for all who have been affected."
University president Charles Steger said the "entire nation and people from many other nations are coming together to grieve" over the tragedy, and he hoped it would help begin the healing process.
There was a sombre atmosphere as students thronged the university's Cassell Coliseum sports centre for the memorial which was screened on the campus TV.
Those present hugged and comforted each other as they left the convocation, sharing a moment of release that many said was badly needed. Looking forward, they said they believed the university's community spirit would pull them through but that there were still tough days ahead.
The shootings on Monday occurred in two separate locations, two hours apart.
The first took place at 0715 (1215 GMT), at West Ambler Johnston Hall dormitory, where a man and a woman were shot dead.
Then, at about 0915 (1415 GMT), 30 people were killed in the second shooting at Norris Hall, about half a mile (800 metres) from the first, on the same campus.
Police confirmed that two handguns - 9mm and .22 calibre weapons - were recovered at the scene of the second shooting, and that one of them was also used in the first incident.
"It's certainly reasonable for us to assume that Cho was the shooter in both places," police said.
However, they are investigating whether he had an accomplice.
Police said Cho Seung-hui was a South Korean living legally in the US.
He came to the US with his parents when he was aged eight.
South Korean President Roh Moo-hyun "was shocked beyond description", and sent "deep condolences" to the victims, their families and the American people, his office said in a statement.
Police did not suggest a motive for the attack.
Larry Hincker, associate vice president for university relations at Virginia Tech, said: "He was a loner, and we're having difficulty finding information about him."
US media reported that a "disturbing" note had been found in Cho Seung-hui's dormitory, and that he had been referred for counselling after producing "troubled" work in his creative writing class. But police say they have no evidence that he left a suicide note.
At a press conference on Tuesday the superintendent of Virginia state police, Colonel Steve Flaherty, described the scene of the massacre.
"Personal effects were strewn about the entire second floor of Norris Hall so it made it much more difficult for us to identify students and faculty members that were victims," he said.
Among the dead were profs Liviu Librescu (L) and GV Loganathan (R)
"Victims were found in at least four classrooms as well as a stairwell. We know that there were a number of heroic events that took place within minutes of this tragedy unfolding.
"The gunman was discovered among several of the victims in one of the classrooms. He had taken his own life."
At least fourteen people remained in hospital on Tuesday. A hospital spokesman said most were stable.
The university said all classes would be suspended for the rest of the week, while Norris Hall would remain closed for the rest of the semester.
Some students have complained that they were put at risk by the university, saying they had received no warning until an e-mail more than two hours after the first incident.
WORST US SCHOOLS SHOOTINGS
1 August 1966 - Sniper Charles Whitman kills 14 people and injures dozens at University of Texas
20 April 1999 - Two teenagers at Columbine High School, Colorado, kill 13 before killing themselves
21 March 2005 - A teenager on an Indian reservation in Red Lake, Minnesota, kills nine
Student Billy Bason, 18, said: "I think the university has blood on their hands because of their lack of action after the first incident."
But the university president has defended his staff, saying they "had no reason to suspect any other incident was going to occur".
Virginia Tech and police said they would not name the victims of the attacks until all had been identified, and their families informed.
However, some of the names of the victims emerged when their families or colleagues volunteered the information.
At least two of the dead were university professors.
One was GV Loganathan, 51, an Indian-born lecturer in civil and environmental engineering, and another was Liviu Librescu, a Romanian-born Israeli expert on aeronautical engineering.