Witnesses in the geology class said "someone dressed in black came out from behind a screen in front of the classroom and opened fire with a shotgun," Peters said.
Lauren Carr said she was sitting in the third row of the lecture hall around 3 p.m. when she saw the shooter walk through a door on the right-hand side of the stage, pointing a gun straight ahead.
"I personally Army-crawled halfway up the aisle," said Carr, a 20-year-old sophomore. "I said I could get up and run or I could die here."
She said a student in front of her was bleeding, "but he just kept running."
"I heard this girl scream, 'Run, he's reloading the gun."'
Peters said the gunman was a former graduate student in sociology at NIU, but was not currently enrolled at the 25,000-student campus about 65 miles west of Chicago.
"It appears he may have been a student somewhere else," University Police Chief Donald Grady said, adding that police had no apparent motive.
Seventeen victims were brought to Kishwaukee Community Hospital in DeKalb, according to spokeswoman Theresa Komitas. One died, two were admitted and three were discharged; five are being evaluated and six others were transferred to other hospitals in critical condition. At least one male died at OSF St. Anthony Medical Center in Rockford, an official said.
George Gaynor, a senior geography student, who was in Cole Hall when the shooting happened, told the student newspaper the Northern Star that the shooter was "a skinny white guy with a stocking cap on."
He described the scene immediately following the incident as terrifying and chaotic.
"Some girl got hit in the eye, a guy got hit in the leg," Gaynor said outside just minutes after the shooting occurred. "It was like five minutes before class ended too."
Witnesses said the young man carried a shotgun and a pistol. Student Edward Robinson told WLS that the gunman appeared to target students in one part of the lecture hall.
"It was almost like he knew who he wanted to shoot," Robinson said. "He knew who and where he wanted to be firing at."
Jillian Martinez, a freshman from Carpentersville, told the Chicago Tribune she was in the auditorium when the gunman entered through a door to the right of the lectern and opened fire about 3 p.m. "He just started shooting at all the kids," she said. "He just started shooting at people, and I ran out of there as fast as I could. I ran all the way to the student center; when I got there I could still hear shooting (from the classroom).
Agents with the U.S. Bureau of Alcohol, Tobacco, Firearms and Explosives were assisting local authorities at the scene, spokesman Thomas Ahern told the Chicago Tribune.
"We will be urgently tracing the firearms and learning the history of the weapons," Ahern said.
All classes were canceled Thursday night and the campus was closed on Friday. Students were urged to call their parents "as soon as possible" and were offered counseling at any residence hall, according to the school Web site.
The school was closed for one day during final exam week in December after campus police found threats, including racial slurs and references to shootings earlier in the year at Virginia Tech, scrawled on a bathroom wall in a dormitory. Police determined after an investigation that there was no imminent threat and the campus was reopened. Peters said he knew of no connection between that incident and Thursday's attack.
The shooting was the fourth at a U.S. school within a week.
On Feb. 8, a woman shot two fellow students to death before committing suicide at Louisiana Technical College in Baton Rouge. In Memphis, Tenn., a 17-year-old is accused of shooting and critically wounding a fellow student Monday during a high school gym class, and the 15-year-old victim of a shooting at an Oxnard, Calif., junior high school has been declared brain dead.