Story Highlights• Witness describes gunman coming back to classroom
• University officials say 33 dead, including a gunman
• Attacks mark deadliest mass shooting in U.S. history
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BLACKSBURG, Virginia (CNN) -- A gunman who killed at least 30 people in one of two shootings on the campus of Virginia Tech was dressed "almost like a Boy Scout," said a student who survived by pretending to lie dead on a classroom floor.
"He just stepped within five feet of the door and just started firing," said Erin Sheehan, who was in one of the Norris Hall classrooms where the second shooting incident took place.
Sheehan described the gunman -- who later shot and killed himself, according to police -- as a young man wearing a short-sleeved tan shirt and black ammunition vest.
"He seemed very thorough about it -- getting almost everyone down -- I pretended to be dead," she said. (Watch student describe surviving by playing dead )
"He was very silent," said Sheehan, one of only four students in her 25-student German class who were not shot.
The gunman left but returned in about 30 seconds. "I guess he heard us still talking," said Sheehan.
"We forced ourselves against the door so he couldn't come in again, because the door would not lock."
The man tried three more times to force his way in and then began firing through the door, she said.
Student Tiffany Otey was taking a test inside Norris Hall when the shooting began. She and about 20 other people took refuge behind a locked door in a teacher's office.
Police officers with bulletproof vests and machine guns were in the area.(Watch a student's recording of police responding to loud bangs )
"They were telling us to put our hands above our head and if we didn't cooperate and put our hands above our heads they would shoot," Otey said. "I guess they were afraid, like us -- like the shooter was going to be among one of us." (Watch students react to shooting )
Some students leaped from windows to escape, said Matt Waldron.
"These two kids, I guess, had panicked and jumped out of the top-story window, and the one kid broke his ankle and the other girl was not in good shape just lying on the ground."
Madison Van Duyne said she and her classmates in a media writing class were on "lockdown" in their classroom, where they where writing about the shootings and posting online.
Shooter chained doors shut
University President Charles Steger told reporters Monday night that police found the front doors of Norris Hall chained shut and that by the time they got to the second floor, the gunfire stopped. (Watch gunfire on the campus )
The shooter attacked more than one classroom at Norris Hall, according to police. The death total there -- 31 including the gunman -- makes it the deadliest shooting in U.S. history.
A law enforcement source close to the investigation told CNN a .22-caliber handgun and a 9 mm handgun were recovered at the scene.
University police Chief Wendell Flinchum said at a Monday night news conference that authorities had a preliminary identification of the shooter at Norris Hall but were not releasing it.
Dormitory shooting two hours earlier
Two people were killed in a separate incident at a dormitory on the campus about two hours before the Norris Hall incident, at around 7:15 a.m.
The dormitory, West Ambler Johnston Hall, houses 895 students and is located near the drill field and stadium. (Campus map)
Flinchum said police were still investigating whether the two incidents are related.
At a news conference Monday afternoon, Flinchum did not rule out a separate shooter for the dormitory incident. (Watch the police chief explain where bodies were found )
At the time of the later shootings at Norris Hall, police were investigating a "person of interest" in the dormitory shootings, Flinchum said. But the man -- a non-student who knew one of the victims -- had not been arrested, and it is unclear if he has any link to the other gunman, he said.
One dormitory victim identified
Courtney Dalton, an 18-year-old student who worked at West End Dining Hall, said a friend named Ryan Clark was one of the two dormitory victims.
She said Clark, a resident assistant at West Ambler Johnston Hall, had once worked at the cafeteria serving pizza.
"He was a happy person; this is really sad," she said, sobbing.
"All I can do is pray for his family now," she told CNN.com.
Officials thought first incident was isolated
Asked why the campus, which has more than 26,000 students, was not shut down after the first shooting, Flinchum responded that police determined "it was an isolated event to that building and the decision was made not to cancel classes at that time." (Officials thought shooter had fled)
The university has scheduled a convocation for 2 p.m. ET Tuesday. Classes also have been canceled Tuesday. In Washington, the House and Senate observed moments of silence for the victims and President Bush said the nation was "shocked and saddened" by news of the tragedy.
Last August, the first day of class was cut short at Virginia Tech by a manhunt for an escaped prisoner accused of killing a Blacksburg hospital security guard and a sheriff's deputy.
Before Monday, the deadliest mass shooting in the United States occurred in 1991, when George Hennard drove a pickup truck into a Killeen, Texas, cafeteria and fatally shot 23 people, before shooting and killing himself.
CNN's Ashley Fantz and Jeanne Meserve contributed to this report.
INFORMATION Concerned parents should call the dean of students' office at 540-231-3787.
Anyone with information about the shootings should call the Virginia Tech Police Department at 540-232-8477.
VICTIMS TAKEN TO HOSPITALS
Montgomery Regional Hospital
Treated 17 patients
1 pronounced dead on arrival
3 in critical condition
6 in stable condition
2 transferred to other facilities
Lewis-Gale Medical Center
Lewis-Gale Medical Center
Treated 5 patients
3 in stable condition; 1 underwent surgery