A black-clad former student turned a university lecture hall into a Valentine's Day massacre on Thursday, killing five people and injuring 16 others before turning a gun on himself, in the fifth US school shooting in a week, authorities said.
Armed with a shotgun and two handguns, he calmly stepped out from behind the curtain at the front of an auditorium just minutes before a geology class ended, officials and witnesses said.
Screams filled the hall as he sprayed a hail of bullets from the stage of the auditorium filled with dozens of students at Northern Illinois University, in a suburb of Chicago.
"It started and ended within a matter of seconds," university police chief Donald Grady said at a press conference.
Police were in the room within two minutes of getting the call but did not have time to fire a single shot as the gunman was already dead on the stage.
"We have no apparent motive at this time," Grady added.
Witnesses described the shooter as a white male dressed in black around six-foot (1.8 meters) tall.
"He was aiming towards the crowd but I don't think he was aiming at a specific person," a witness named Sheila told WBBM radio.
"He was quiet. He just stood on the stage in front of everybody and just started shooting.
"I saw him holding the gun and it was huge. I thought it was fake and then I realised he was really shooting at people and I got down," she said. "I saw a lot of blood. I have blood all over my clothes."
All of those shot were students, including the instructor who is a graduate student, university president John Peters said.
The shooter killed four women and one man, three of whom were dead by the time paramedics arrived, he added.
Six victims remain in critical condition while eight were discharged within a few hours, hospital officials said.
The shooter was enrolled as a graduate student in sociology last year and then transferred to another state institution, officials said.
"The information we have right now indicates he did not have any record of police contact or a prior arrest record," Peters told a press conference.
The massacre follows school shootings in Ohio, Louisiana, Tennessee and California that left a total of five dead.
It comes 10 months after 32 students and faculty were shot down by a mentally disturbed student at Virginia Tech University in the deadliest massacre ever at a US school.
Northern Illinois University was placed on a security alert in December after police found threats on a bathroom wall laced with racial slurs, references to the Virginia Tech shooting and a warning that "things will change most hastily" in the final days of the semester.
But Peters said he did not think Thursday's shooting was related to the threats and said that while security has been heightened there was not much more officials could have done.
"I don't know of any plan that can prevent this tragedy," he told reporters. "Unless we lock every door I don't know how we can keep people out."
Students described now-familiar scenes of panic spreading across campus.
"I saw a lot of confusion," said Dominique Broxton, 22, describing the scene from her dorm room. "Students were running. People really didn't know what was going on."
Broxton said she could see two wounded students from her dorm room.
"The ambulance took away two students on the ground right outside my dorm," she said. "I don't know them. They looked bloody. Where I am right now, there are a lot of police, at least a dozen. There are police cars and trucks everywhere."
"There is an intercom system inside the dorm. Someone came on and stated that someone had been caught. They said they caught the shooter and that we should remain calm and stay in our rooms."
Chicago has long been noted for the Valentine's Day Massacre of February 14, 1929, when seven people were executed by machine-gun in a Mafia killing during the city's gangster heyday.
Northern Illinois University, chartered in 1895, is a teaching and research institution with a student enrollment of more than 25,000 and nearly 1,300 teachers. It has 862 international students from 88 nations. (AFP)
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