A former student dressed in black walked onto the stage of a lecture hall at Northern Illinois University and opened fire on a packed science class, killing five students, wounding 16 and setting off a panicked stampede before committing suicide. Police say they have no motive for the rapid-fire assault, carried out Thursday afternoon by the gunman, who fired indiscriminately into the crowd with a shotgun and two handguns as students dove to the floor and ran toward the exit. At least two of the wounded were hospitalized in critical condition. The shooting was the fourth at a US school within a week. "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 student 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." University President John Peters said four people died at the scene, including three students and the gunman, while the other two died at a hospital. The teacher, a graduate student, was wounded but was expected to recover. Peters said the gunman was a former graduate student in sociology at NIU, but 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. Witnesses said the skinny gunman, dressed in black and wearing a stocking cap, emerged from behind a screen on the stage of 200-seat Cole Hall and opened fire just as the class was about to end around 3 p.m. 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. "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." Seventeen victims were brought to nearby Kishwaukee Community Hospital, where one died, according to spokeswoman Theresa Komitas. One male was transferred in critical condition and died at OSF St. Anthony Medical Center in Rockford, an official said. Minutes after the shooting erupted, students phoned each other and sent text messages even before school officials could warn them, many said. The school Web site announced a possible gunman on campus within 20 minutes of the shots and locked down the campus, part of a new security plan created after a student at Virginia Tech killed 32 people last year. "This is a tragedy, but from all indications we did everything we could when we found out," Peters said. George Gaynor, a 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 told WLS that 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 on Friday. 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. Peters said he knew of no connection between that incident and Thursday's attack. In the past week, there have been four shootings at US schools. On Feb. 8, a woman shot two fellow students to death before committing suicide at Louisiana Technical College in Baton Rouge. In Memphis, Tennessee, 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, California, junior high school has been declared brain dead. Also, on Feb. 7, a man opened fire on a city council meeting in the St. Louis suburb of Kirkwood, killing five people before he was shot dead by police.