The campus of Northern Illinois University looked like a ghost town just hours after a lone gunman, armed with a shotgun and two handguns, opened fire on students in a lecture hall.

The gunman, whose identity had not been released, killed five students and took his own life, officials said.

Students fled campus for safety in the aftermath of sirens, screams and fear.

"All of a sudden, you just heard a bunch of screaming and everybody just came rushing out like a big stampede," said Katie Scott, an NIU freshman. "There were people with blood on them; there were people with other people's blood on them. It was a worst nightmare."

Scott and her friend, Nicole Parisi, also a freshman, were leaving campus.

Carrying a stuffed animal and pillow, Parisi said the shooting was "unbelievable."

University President John Peters called the shooting a "terrible time of crisis" on campus. Flags were lowered to half staff in honor of the fallen students.

At 3:06 p.m., the first 911 calls rolled in. Minutes later, police arrived on the scene.

Twenty-one students were shot and the gunman lay dead. Investigators were searching for clues of why the gunman, who was dressed in black, chose to carry out the massacre.

Herminia Irizany, a senior and editor at the Northern Star student newspaper, said students were trying to comprehend what happened.

"From what I've seen, people are just scared."