Survivors of the worst shooting rampage in US history have accused authorities of not acting quickly enough to alert people a gunman was on the loose.
The unidentified gunman killed 32 people at Virginia Tech university, before committing suicide.
There were two attacks two hours apart, with 30 people shot dead in the second.
Students have said they were not properly warned after the first shooting, but officials said they acted on what they knew at the time.
President George W Bush said the US was "shocked and saddened" by the attack at the university, home to 26,000 students, in the town of Blacksburg.
Many unanswered questions remain about the attacks.
Police have still not said for sure that the two attacks were linked. They have confirmed that two handguns were recovered.
They say they have a preliminary ID of the dead gunman but have not released it. There is no clue yet as to any motive.
Virginia Tech police chief Wendell Flinchum also refused to rule out the possibility of a co-conspirator.
"I'm not saying there is someone out there, and I'm not saying there is someone who is not," he said.
One witness, Erin Sheehan, described the gunman as "a little bit under six-feet tall, young looking, Asian, dressed sort of strangely, almost like a boy scout, very short-sleeved light, tan shirt and some sort of ammo vest with black over it".
Some students complained angrily that they had received no warning from the university until an e-mail more than two hours after the first incident at 0715 (1215 GMT), at West Ambler Johnston Hall dormitory.
The message said a shooting had occurred and urged students to be cautious.
A second e-mail was sent apparently after the second shooting, at the Norris Hall engineering building, nearly half a mile (800 metres) away, was over.
VIRGINIA TECH UNIVERSITY
Founded in 1872
26,000+ students and 10,000 staff
2,600-acre (1,040-hectare) campus
It read: "A gunman is loose on campus. Stay in buildings until further notice. Stay away from all windows."
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 Virginia Tech President Charles Steger said: "We had no reason to suspect any other incident was going to occur.
"We can only make decisions based on the information you had at the time. You don't have hours to reflect on it."
Police said they thought the first incident was isolated and "domestic" and that the gunman had left the campus.
Eyewitnesses said some students jumped from classroom windows to escape the gunfire, which triggered panic on campus.
Some of those inside the university buildings were using the internet to try to glean information about what was happening and many e-mailed the BBC News website.
Nikolas Macko described how his class in Norris Hall barricaded a door against the gunman.
"The shooter tried to open the door, but my classmates kept it well shut, as they held the table against it from floor level.
"The shooter shot the door twice at chest level, which resulted in two holes in the door, one of which hit the podium in the front of the class room and the other continued out the window. At this point he reloaded, shot the door again - this shot did not penetrate - and moved on to the other classrooms," Mr Macko said.
The university was closed on Tuesday, and officials said counsellors were on site for students.
The victims have not been officially named, although the identities of some of the dead have emerged.
Two of those killed were prestigious academics - 75-year-old Romanian-born Professor Liviu Librescu, an Israeli citizen and one of the world's leading academics in the field of aeronautical engineering, was among those shot dead, his family said.
Indian-born award-winning Professor G V Loganathan, 51, a lecturer in civil and environmental engineering, was also killed, his relatives confirmed.
The deadliest mass US shooting prior to the Virginia attack was in Texas in 1991 when George Hennard killed 23 people and himself in a cafeteria.
The US also has a history of deadly school shootings.
In 1966, the day after killing his wife and mother, gunman Charles Whitman opened fire from a tower on the campus of the University of Texas killing 14 people and injuring 31 others.
In 1999 two teenagers at Columbine High School in Colorado killed 12 fellow students and a teacher before taking their own lives.
LOCATIONS OF VIRGINIA TECH CAMPUS SHOOTINGS
1) 0715: Two killed at West Ambler Johnston Hall, a dormitory building. Police believe incident isolated and "domestic" in nature
2) 0915 (approx): 30 killed in shooting at engineering building, Norris Hall. Gunman kills himself