Police have identified a gunman who killed five students when he opened fire at a university near Chicago as former student Stephen Kazmierczak.
Police say Kazmierczak's behaviour had become erratic recently
The head of police at Northern Illinois University, Don Grady, said Kazmierczak had studied sociology there in 2007.
Mr Grady said it was not known why Kazmierczak, 27, who was armed with three handguns and a shotgun, opened fire in a crowded lecture hall.
Another 15 people were injured before Kazmierczak killed himself.
Student Desiree Smith, who was in the hall, said the gunman suddenly appeared 10 minutes before the end of the lecture.
She said: "He was dressed all in black, he had a black ski cap on and then I saw him shoot.
"He aimed at our professor and I did see our professor shot in the arm."
Mr Grady said there had been nothing to suggest Kazmierczak would commit such an act, but added he had become erratic in recent weeks after he stopped taking some unspecified medication.
"Actually, there were no red flags," the police chief said.
"He was someone that was revered by the faculty and staff and students alike.
The police chief said it was still unclear how many shots had been fired, but 48 bullet casings had been found as well as six shotgun shells.
He praised the actions of the emergency services, saying police had been on the scene within minutes.
The gunman's tearful father, Robert Kazmierczak, told reporters outside his home in Lakeland, Florida, that it was a "very hard time" for him, but was too upset to say more.
The gunman's father wept outside his Florida home
The university's president, John Peters, said the police had rehearsed for such an eventuality.
Mr Peters said there had been an outpouring of support from around the United States and the world following the attack.
US President George W Bush expressed his sorrow at the shootings, saying: "Obviously, a tragic situation on that campus, and I asked our fellow citizens to offer their blessings."
As the university tries to come to terms with killings, counsellors have been arriving from neighbouring colleges to assist in the grieving process. A candlelit vigil is planned for this evening on the campus.
The shooting comes 10 months after 32 students and staff were killed by a student at Virginia Tech in one of the worst shootings ever at a US school.