A man dressed in black opened fire with a shotgun from a stage of a lecture hall at Northern Illinois University on Thursday, injuring as many as 18 people, four critically, before he killed himself, the school's president said.

University President John Peters said witnesses "say someone dressed in black came out from behind a screen in front of the classroom and opened fire with a shotgun."

University Police Chief Donald Grady said the gunman was not a student at the school. "It appears he may have been a student somewhere else," he said, adding that police had no apparent motive.

Grady said the gunman died from a self-inflicted gunshot wound on a stage in the front of the lecture hall. He said the whole incident was over in about two minutes.

NIU said 18 victims had been transported to Kishwaukee Community Hospital in DeKalb, but Joe Dant, a hospital spokesman, said 17 victims with gunshot wounds had arrived at the hospital, reported WMAQ-TV in Chicago.

Three were in critical condition with head wounds, and one of those had been transported to a higher-level care facility in Rockford, Ill. Eight patients were in yellow, or stable, condition with "indirect gunshot wounds," Dant said. Six patients were in green, or good, condition.

The shooting took place in Cole Hall near the King Commons, a central gathering place on the 25,000-student campus, according to the university's Web site. The university is about 65 miles west of Chicago.

NIU professor Allen May told WISN-TV in Milwaukee that the gunman dropped from an overhead position in the lecture hall and took aim at students with a shotgun.

"It would be someone who would have to know the physical layout and architecture of that room," May said. "I don't know how you suddenly magically appear from a rafter area and just drop down onto the main portion of it without having been in that area in that building in that class room before."

May said he assumed most of the injured were students since it took place in a classroom, but he wasn't sure if any non-students were among the injured.

Paul Sundstrom said he was in geography class in the Cole Hall auditorium when the gunfire broke out, WMAQ reported.

"We were sitting in the fourth row and the guy came in from behind where the professor was speaking. He had a black beany, like a long black trench coat and a shotgun, and he just walked in and just started shooting at people randomly.," Sundstrom said. "I crawled out to the main aisle, then just got up and ran and turned around and saw him shooting."

Sundstrom said there were 150 to 200 students in the auditorium when the shooting started.

Tom Ahern, an agent with the Bureau of Alcohol, Tobacco, Firearms and Explosives said the gunman used a 12-gauge shotgun and a handgun. Ahern said agents did not know the make or models of the guns, but they would be analyzed on a high-priority basis.

Campus has been locked down, according to DeKalb County Sheriff Roger Scott. Students have reported police cars and ambulances on campus, with streets blocked off.

A sophomore told WBBM radio that he was sitting in geology class when a young white male took out a 12-gauge shotgun and started shooting in Cole Hall, WMAQ reported.

The student said he ran, but believes a professor may have been shot. That information has not been confirmed.

"I was in class, and everything was normal. A student from a different class came in and said there had been a shooting and everyone had to get out of here," another student, named Noah, told WMAQ.

Noah said his professor had one student go to an administrative office and ask what was happening. The student returned and said that the shooting was confirmed, and that students should leave.

NIU confirmed the shooting in a posting on its Web site.

The university was urging people not to come to campus and all activities were cancelled at the campus for Thursday and Friday.

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.

Grady said campus police didn't have any credible threats prior to Thursday's shooting. He said the university had taken all precautions it could against violence on campus.

"I wish I could tell you there was a panacea for this, but you've seen the reports of shootings like this across the nation," Grady said. "It's very hard to prevent a situation like this."