It appeared that the gunman had either been taken into custody or had been killed.
"Campus police report that the immediate danger has passed," the university's Web site said. "The gunman is no longer a threat."
One local media report said police had the gunman in custody, other media reported he was dead.
A hospital near the campus said it expected to receive up to 15 people wounded from the incident.
A local newspaper said the gunman was armed with a shotgun and a pistol.
One student told local radio that roughly 140 students were in a geology classroom when the man stood up and opened fire. Other witnesses said bloodied victims were hit by buckshot.
The 25,000-student campus, which is 65 miles (104 km) west of downtown Chicago, was shut down and classes cancelled, the university said on its Web site.
A woman who was outside the classroom when the shooting occurred said students fled in terror. "One guy said, there's someone shooting," she said.
U.S. schools and colleges have seen a series of shooting incidents in recent years.
A university in Blacksburg, Virginia, Virginia Tech, became the site of the deadliest rampage in U.S. history in April last year when a gunman killed 32 people and himself.
