DEKALB, Ill. – A former student dressed in black opened fire with a shotgun and two handguns from the stage of a lecture hall at Northern Illinois University on Thursday, killing five students and injuring 16 others before committing suicide, authorities said.

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 died at the scene, including the gunman, and the other two died at a hospital, he said. Two victims were in critical condition.

Investigators did not know what led the gunman, a former NIU graduate student in sociology, to spray bullets at the geology class instructor and dozens of students in the large hall around 3 p.m.

“I kept thinking, ‘Oh God, he’s going to shoot me. Oh God, I’m dead. I’m dead. I’m dead,’ ” said Desiree Smith, a senior journalism major who dropped to the floor near the back of the auditorium.

“People were crawling on each other, trampling each other,” she said. “As I got near the door, I got up and I started running.”

Officials said 162 students were registered for the class but it was unknown how many were there Thursday.

Lauren Carr said she was sitting in the third row when she saw the shooter walk through a door on the right-hand side of the stage, pointing a gun straight ahead.

“I personally Army-crawled halfway up the aisle,” said Carr, a 20-year-old sophomore. “I said I could get up and run or I could die here.”

She said a student in front of her was bleeding, “but he just kept running.”

“I heard this girl scream, ‘Run, he’s reloading the gun.’ “

Student Jerry Santoni was in a back row when he saw the gunman enter a service door to the stage.

“I saw him shoot one round at the teacher,” he said. “After that, I proceeded to get down as fast as I could.”

Santoni dived down, hitting his head on the seat in front of him, leaving a knot about half the size of a ping pong ball on his forehead.

The teacher, a graduate student, was wounded but was expected to recover, the school president said. He did not give details of the injuries.

Peters said the gunman was not currently enrolled at the 25,000-student campus about 65 miles west of Chicago.

“It appears he may have been a student somewhere else,” University Police Chief Donald Grady said.

Seventeen victims were brought to nearby Kishwaukee Community Hospital, where one died, according to spokeswoman Theresa Komitas. School officials said four people, including the gunman, died at the lecture hall and two later died at hospitals.

Michael Gentile was meeting with two of his students directly beneath the lecture hall when the shootings happened. He could hear the chaos a few feet above his head.

“The shotgun blast must have been so loud,” said Gentile, a 27-year-old media studies instructor. “It sounded like something was dropping down the stairs … We had no idea what this was.”

Then, shorter, sharper noises he recognized as handgun shots.

“There was a pretty quick succession … just pow, pow, pow,” said Gentile, who didn’t leave his office for about 90 minutes. He used a surveillance camera just outside his office to confirm that the people knocking on his door were police.

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.”

Witnesses said the young man carried a shotgun and a pistol. Student Edward Robinson said the gunman appeared to target students in one part of the lecture hall.

“It was almost like he knew who he wanted to shoot,” Robinson said. “He knew who and where he wanted to be firing at.”

The federal Bureau of Alcohol, Tobacco and Firearms sent 15 agents to the scene, according to spokesman Thomas Ahern. He said information about the weapons involved would be sent to the ATF’s national database in Washington and given urgent priority. The FBI also was assisting.

All classes were canceled Thursday night and the campus was closed today. Students were urged to call their parents “as soon as possible” and were offered counseling at any residence hall, according to the school Web site.

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. Peters said he knew of no connection between that incident and Thursday’s attack.

The shooting was the fourth at a U.S. school within a week.

On Feb. 8, a woman shot two fellow students to death before committing suicide at Louisiana Technical College in Baton Rouge. In Memphis, Tenn., a 17-year-old is accused of shooting and critically wounding a fellow student Monday during a high school gym class, and the 15-year-old victim of a shooting at an Oxnard, Calif., junior high school has been declared brain dead. That shooting has been called a hate crime.