(MCT) CHICAGO-A former graduate student armed with a shotgun and two handguns opened… (MCT) CHICAGO-A former graduate student armed with a shotgun and two handguns opened fire Thursday afternoon in a lecture hall at Northern Illinois University in DeKalb, Ill., killing five students and wounding 16 others before fatally shooting himself, authorities said.

The shooting occurred about 3 p.m. during a geology class in a large lecture classroom in Cole Hall, authorities said. The gunman, dressed in black, entered through a back door and came through a curtain behind the stage before opening fire on the students.

“The assailant began firing into the assembled class from the stage,” NIU President John G. Peters said at a news conference Thursday evening.

“Eyewitness accounts describe a very brief rapid-fire assault that ended with the gunman taking his own life.”

Four of the victims who were killed were female, and one was male, Peters said. They were all undergraduate students. The wounded included 15 undergraduate students and one graduate student who was acting as a teacher’s assistant, Peters said.

Four, including the gunman, died at the scene, and two died later in a hospital, Peters said. As of 8 p.m., four patients were reportedly in critical condition, two at Kishwaukee Community Hospital in DeKalb and two at other regional hospitals.

The gunman was a sociology graduate student who was enrolled in classes at NIU last spring but was not currently enrolled there, Peters said.

Authorities were not releasing his identity Thursday night but said they were not aware of any criminal history or violence in his past.

The gunman was found dead on the stage of the lecture hall, NIU Police Chief Donald Grady said. The shooter had a shotgun, a Glock pistol and one other small-caliber handgun, with ammunition still left in both handguns, Grady said. He said gun magazines were found “all over the floor.”

“We believe there was only one shooter,” Grady said. The shooting was “over in a matter of minutes,” he said.

Peters said the motive for the “senseless tragedy” was unknown, but authorities had “no reason to believe” it was related to threats found on a bathroom wall on campus in December. The threats made reference to the 2007 massacre at Virginia Tech.

Jillian Martinez, a freshman, said she was in the auditorium in Cole Hall 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).”

Martinez said the assailant was a white man and was carrying a large gun.

Sophomore Geoff Alberti told his parents he was in the geology class when the gunman entered the auditorium-style classroom through an emergency exit. The gunman did not say anything before opening fire on the class, he told his parents.

“He said at least 20 rounds were fired,” said his mother, Marilyn Alberti.

Most students dropped to the floor after hearing the first shot, then crawled out of the classroom on their bellies, she said. Geoff Alberti called his parents at 3:20 p.m. immediately after exiting the building.

“He was just frantic,” Marilyn Alberti said. “He said, ‘Mom, a guy just shot up my class.'”

In December, the university was placed under a security alert through the end of the semester after police found threats on a bathroom wall that included a racial slur and references to the Virginia Tech shootings.

Two separate messages were found by a student on a restroom wall in the Grant Towers D complex that read that “things will change most hastily” in the final days of the semester, university officials said at the time.

-By Jason Meisner, Jeremy Gorner and Tina Shah, Chicago Tribune

Chicago Tribune reporter Jo Napolitano contributed to this report