(CNN) -- The Virginia Tech police chief said at least 20 people were killed in twin shootings on the Blacksburg campus Monday morning.
"Some victims were shot in a classroom," Chief Wendell Flinchum said, adding that the gunman was dead.
"Today the university was struck with a tragedy that we consider of monumental proportions," said university President Charles Steger. "The university is shocked and indeed horrified." (
Map of Blacksburg)
The attacks mark the worst school shooting incident since 1999 when Dylan Klebold and Eric Harris killed 12 students and a teacher before killing themselves at Columbine High School in Littleton, Colorado.
A hospital spokeswoman told The Associated Press that 17 Virginia Tech students were being treated for gunshot wounds and other injuries.
Sharon Honaker at the Carilion New River Valley Medical Center told CNN that four patients had been transported there, one in critical condition.
One person was killed and others were wounded at multiple locations inside a dormitory about 7:15 a.m., Flinchum said. Two hours later, another shooting at Norris Hall, the engineering science and mechanics building, resulted in multiple casualties, the university reported. (
Watch police, ambulances hustle to the scene 
)
The first reported shooting occurred at West Ambler Johnston Hall, a co-ed dormitory that houses 895 students. The dormitory, one of the largest residence halls on the 2,600-acre campus, is located near the drill field and stadium.
Amie Steele, editor-in-chief of the campus newspaper, said one of her reporters at the dormitory reported "mass chaos."
The reporter said there were "lots of students running around, going crazy, and the police officers were trying to settle everyone down and keep everything under control," according to Steele.
Kristyn Heiser said she was in class about 9:30 a.m. when she and her classmates saw about six gun-wielding police officers run by a window.
"We were like, 'What's going on?' Because this definitely is a quaint town where stuff doesn't really happen. It's pretty boring here," said Heiser during a phone interview as she sat on her classroom floor.
Student Matt Waldron said he did not hear the gunshots because he was listening to music, but he heard police sirens and saw officers hiding behind trees with their guns drawn.
"They told us to get out of there so we ran across the drill field as quick as we could," he said.
Waldron described the scene on campus as "mayhem." (
Watch a student's recording of police responding to loud bangs )
"It was kind of scary," he said. "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 classrooms. They were huddled in the middle of the classroom, writing stories about the shootings and posting them online.
The university is updating its 26,000 students through e-mails, and an Internet webcam is broadcasting live pictures of the campus.
The shootings came three days after a bomb threat Friday forced the cancellation of classes in three buildings, WDBJ in Roanoke reported. Also, the 100,000-square-foot Torgersen Hall was evacuated April 2 after police received a written bomb threat, The Roanoke Times reported.
After the Monday shootings, students were instructed to stay indoors and away from windows, police at the university said.
"A gunman is loose on campus. Stay in buildings until further notice. Stay away from all windows," read a warning from the university.
"Virginia Tech has canceled all classes. Those on campus are asked to remain where they
