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. Four of the six total dead died at the scene, and the other two died at a hospital, he said. Two of the wounded remain in critical condition.
The university president says witnesses in the geology class saw someone come out from behind a screen in front of the classroom and opened fire with a shotgun.
All classes were canceled Thursday night and the 25,000-student campus was closed until further notice.
As many as 16 more were injured before the shooter killed himself, the school's president said.
The shootings took place around 3 p.m. in Cole Hall near the King Commons, a central gathering place on the 25,000-student campus, according to the Web site.
A Kishwaukee Community Hospital spokeswoman said it received 17 victims, all with wounds from the shooting or flying debris, including three with serious injuries. One was airlifted to another hospital.
The (DeKalb) Daily Chronicle cited information on a police scanner, alerting police about a white male shooter with a shotgun and pistol.
Peters said the gunman was a former graduate student in sociology at NIU, but was not currently enrolled at the campus, located 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.
A note on the campus web site posted at 4:10 p.m. read: "Campus police report that the scene is secure. Only essential personnel should remain on campus."
The Web site also said that counseling has been arranged for those on campus who want it.
At 6:30 p.m., word came from Northern Illinois University officials that the gunman had shot and killed himself.
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."