The gunman fatally shot four women and a man in a "brief, rapid-fire assault" that sent terrified students running for cover, university President John Peters said.
Peters said four people died at the scene, including three students and the gunman, while the other two died at a hospital. One of the wounded was the teacher, a graduate student, who was wounded but was expected to recover.
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, according to Peters.
The shooting was the fourth at a U.S. school within a week.
"At this point, I'm being told it was less than two minutes. This thing started and ended in a matter of seconds," Peters said, adding that police had no apparent motive.
Witnesses in the 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.
The gunman shot himself on the stage in front of panicked students, reports CBS affiliate WBBM-TV.
Desiree Smith, a senior journalism major who was in the science class attacked, said students crawled on each other to get away from the shooter.
Minutes after the shooting erupted, many students say they phoned each other and sent text messages even before school officials could warn them. The school Web site announced a possible gunman on campus within 20 minutes of the shots and locked down the campus.
Peters said that the shooter was a former graduate student in sociology at NIU in the spring of 2007, but was not currently enrolled at the 25,000-student campus about 65 miles west of Chicago. He added that the gunman may have been enrolled at another Illinois state institution. Based on the preliminary investigation, it did not appear that the man had any prior arrest record.
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 student. "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."'
Witness Jerry Sandoni was in the classroom at the time the shooting took place. He told the NIU newspaper, The Northern Star, that the gunman was not in the class the entire time but entered toward the end of class.
"He came in from the door on the stage and just started shooting. He didn't say anything."
Sandoni described the shooter as a white male wearing a black shirt and said he had tattoos on his forearms.
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."
Northern Illinois students are talking about threats scrawled on a bathroom wall last December that include racial slurs and references to the Virginia Tech shootings earlier in the year, reports CBS News correspondent Cynthia Bowers. The campus was shut down for a day while university police determined there was no imminent threat. There is no word yet on if there is any connection with today's shooting.
CBS News anchor Katie Couric spoke over the phone to a witness who was in the lecture hall when the gunman entered. She asked to only be identified by her first name, "Katie."
"He started shooting. I just immediately dropped down to the ground," the student, Katie, said.
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.
He said he did not know whether the shooter was a student or what his motive might have been.
"We will be urgently tracing the firearms and learning the history of the weapons," Ahern said.
All classes were canceled Thursday night and the 25,000-student 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.
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.