A US shooting rampage at the Virginia Tech university has left 33 people, including a suspected gunman, dead.
There were two incidents two hours apart, at a student dorm where two were killed and at an engineering building where 30 and the gunman died.
Officers said they were working to link the attacks and had a preliminary ID of the gunman but would not release it.
After the deadliest shooting rampage in US history, President George W Bush said the US was "shocked and saddened".
"Schools should be places of safety and sanctuary and learning. When that sanctuary is violated, the impact is felt in every American classroom and every American community," he said.
The state university in the town of Blacksburg is home to 26,000 students.
Virginia Tech police chief Wendell Flinchum said that emergency services had received a call at 0715 (1215 GMT) alerting them to a shooting at the dormitory - West Ambler Johnston Hall.
He said that two hours later there was a second report of shooting, this time at the engineering building, Norris Hall.
Asked why the campus was not closed after the first shooting, Mr Flinchum said that, at that stage, it was thought to be an isolated incident.
Police believed the first shooting may have been a "domestic incident" and that the gunman had left the campus.
'Many, many shots'
Eyewitnesses said some students jumped from classroom windows to escape the gunfire, which triggered panic on campus.
Some of those locked down 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 was in a mathematics class in Norris Hall when he heard a series of loud bangs in the hallway which prompted a female student sitting near the door to move to close it.
"She peeked out into the hallway, and saw the shooter, so she immediately closed the door. Three other students moved a table that was in front of the room - it seats approximately 40 students at capacity - and barricaded it against the door.
"A few seconds later, 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.
Virginia Tech student Erin Sheehan said she survived an attack on her German class and described the gunman.
She said: "He was, I would say, about 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."
Mr Flinchum said it was unclear if the dead gunman was a student.
He could not confirm that the man was involved in the first attack.
Virginia Tech President Charles Steger said: "Today the university was struck with a tragedy that we consider of monumental proportions."
He said the university was in the process of informing the next of kin of those killed and that counsellors were in place at the campus for student families.
The university urged students to call parents to let them know they were safe.
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.
1) 0715: Two killed at West Ambler Johnston Hall, a dormitory building. Police believe incident isolated and "domestic" in nature
2) 0945 (approx): 30 killed in shooting at engineering building, Norris Hall. Gunman kills himself