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, University President John Peters said.
"It appears he may have been a student somewhere else," University Police Chief Donald Grady said. "We have no motive and I have no way of knowing what the motive was."
Authorities did not release the gunman's name, but Peters said he had no record of police contact or an arrest record while attending Northern Illinois, about 65 miles west of Chicago.
DeKalb County Coroner Dennis J. Miller on Friday 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 transfered to hospitals in other counties, Miller said in a news release.
He said the gunman's name is not yet being released. The gunman died at the scene.