Police said the man was a student from another college but did not publicly identify him.
Donald Grady, campus police chief at Northern Illinois University in DeKalb, about 100km west of Chicago, said there was "no apparent motive at this time".
"It started and ended within a matter of seconds," Grady said.
John Peters, the university president, ruled out the possibility that Thursday's shooting was related to racial threats found on a bathroom wall at the university in December.
The writings on the wall also referred to the Virginia Tech shooting and a warning that "things will change most hastily" as the semester came to an end.
Peters said while security was heightened at the campus there was nothing more officials could have done.
Witnesses said the shooter, a white male dressed in black, burst into the lecture hall with more than 100 students and began randomly firing a shotgun.
"He was aiming towards the crowd but I don't think he was aiming at a specific person," a witness told WBBM radio.
"He was quiet. He just stood on the stage in front of everybody and just started shooting."
Students, some bleeding, fled the classroom.
George Gaynor, a geography student who was in the hall when the shooting happened, told the Northern Star student newspaper 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,'' he said.
All classes were cancelled on Thursday night and Friday at the 25,000-student campus and students were urged to call their parents "as soon as possible''.
"People are urged not to come to campus," the university's website said.
Source: Al Jazeera and agencies