Polk County, Fla., sheriff's officials said they were asked to speak with "the father of the shooting suspect" — Robert Kazmierczak of Lakeland, Fla.

The gunman is the younger Kazmierczak, a university official told The Associated Press on condition of anonymity because the identity has not been released by police.

The motive of the killer, who graduated from NIU in 2006, was still not known as the investigation unfolded Friday, officials said. The gunman also wounded 15 people in Thursday's attack, which sent panicked students fleeing for the exits.

"There is no note or threat that I know of," NIU President John Peters said on Friday ABC's "Good Morning America." "By all accounts that we can tell right now (he) was a very good student that the professors thought well of."

DeKalb County Coroner Dennis J. Miller released the identities of the four victims who died in his county: Daniel Parmenter, 20, of Westchester; Catalina Garcia, 20, of Cicero; Ryanne Mace, 19, of Carpentersville; and Julianna Gehant, 32, of Meridan.

Two other victims died after being transferred to hospitals in other counties, Miller said. Winnebago County Coroner Sue Fiduccia said a female victim died in her jurisdiction but has not been identified pending notification of family.

Witnesses said the 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.

Allyse Jerome, 19, a sophomore from Schaumburg, said the gunman burst through a stage door and pulled out a gun.

"Honestly, at first everyone thought it was a joke," Jerome said. Everyone hit the floor, she said. Then she got up and ran, but tripped. She said she felt like "an open target."

"He could've decided to get me," Jerome said. "I thought for sure he was gonna get me."

The shooter had been a graduate student in sociology at Northern Illinois as recently as spring 2007, but was not currently enrolled at the 25,000-student campus, Peters said. He also said the suspect had no record of police contact or an arrest record while attending Northern Illinois, about 65 miles west of Chicago.