CHICAGO -- On the Northern Illinois University campus, Stephen P. Kazmierczak, 27, was considered a gentle, hard-working student, who was honored two years ago with a dean's award for his sociology work.

Professors who taught him said it was hard to imagine he was the same person authorities identified as the gunman in Thursday's classroom shootings.

"I knew Steve both as an undergraduate and as a graduate student. I have had him in my home. I knew him as a warm, sensitive, very bright student," wrote Professor Kristen Myers in an e-mail. "I never would believe that he could do this. I know that when these horrible things happen, everyone searches for roots to explain it. Here, I'm afraid I don't have any."

Authorities in DeKalb County confirmed yesterday that Kazmierczak had recently stopped taking medication.

No further details were available.

NIU President John G. Peters said Kazmierczak, who was accused of killing five people Thursday before taking his own life, had "a very good academic record, no record of trouble," while studying at the university.

University officials and former teachers said Kazmierczak had established himself as an authority on prison systems, having co-written manuscripts on self-injury in prison and religion's role in the formation of early prisons in the United States.

Both papers were written under the guidance of Jim Thomas, a professor emeritus at NIU and a nationally renowned criminal-justice expert.

Thomas first became acquainted with Kazmierczak when the student took an introductory sociology course, taught in Cole Hall - where Thursday's shooting took place.

"In this large class, he stood out. So I tried to use him as an unpaid assistant," Thomas said. "He stood out because he was hard-working, he was bright, he would come up and talk about ideas behind what I'd taught."

Thomas said he was left dumbfounded when news of the gunman's identity trickled out around campus Thursday.

"When I heard yesterday that it was a student in corrections and social justice, a former grad student, I thought, 'Oh, my God, that's Steve. That has to be Steve,'" he said. "It's nuts, nuts, totally nuts. He was the most gentle, even guy."

One former classmate called Kazmierczak "probably the best student in the class."

The classmate, who asked not to be identified, said Kazmierczak routinely followed the news and often talked about what was on The Colbert Report and The Daily Show with Jon Stewart.

"It surprises me that he would have had a gun," the classmate said.

Thomas said Kazmierczak left the university after the spring 2007 semester to transfer to the University of Illinois.

At the time, NIU colleagues were troubled because Kazmierczak failed two classes and received an incomplete in another course. Thomas, however, believed Kazmierczak failed the classes because he had already shifted his focus to the University of Illinois.

Thomas said the only negative feelings he ever got from Kazmierczak were about NIU's scaling back its criminology program. "So, he felt like his academic needs weren't being met," Thomas said.

After he left DeKalb, Kazmierczak worked briefly as a guard at an Indiana prison about 80 miles from Urbana-Champaign, Ill.

Doug Garrison, a spokesman for the Indiana Department of Corrections, said Kazmierczak worked at the Rockville Correctional Facility in western Indiana from Sept. 24 to Oct. 9, 2007.

Then he abruptly left.

"He just did not come back to work," Garrison said. "He called up one day and said he was not coming back."

Garrison said he did not know why Kazmierczak quit. Garrison declined to discuss any aspect of Kazmierczak's job application.

Thomas said most of his communication with Kazmierczak in the last several months was by e-mail. When the professor asked him why he quit his job at the prison, he said, "he never really responded."

While in Champaign, Kazmierczak lived with his former girlfriend on the city's west side, several miles from campus. Thomas said the couple was no longer romantically involved but had continued to share an apartment.

Authorities in DeKalb confirmed yesterday that Kazmierczak had recently stopped taking medication. Thomas said Kazmierczak had confided in him that he had served in the military and received a discharge for psychological reasons.

"He only discussed that with me in passing," Thomas said. "He seemed as normal as you or I."

Kazmierczak grew up in Elk Grove Village, Ill., but records show his parents moved to Florida shortly before his mother's death in 2004.

Kazmierczak graduated from Elk Grove High School in 1998 with a B average, spokeswoman Venetia Miles said. While at the school, he participated in band, the Japanese language program and a public service class.

Miles said most of the teachers and counselors at the school during Kazmierczak's time there have left or retired. She said most teachers and staff still at the school only have vague recollections of him.

Kazmierczak's father, Robert Kazmierczak, gave a brief comment outside his home in Lakeland, Fla., about 50 miles southwest of Orlando. "Please leave me alone," he said. "I have no statement to make. This is a very hard time. I'm a diabetic."

He then broke down crying and went back inside.

With him were several deputy sheriffs from the Polk County sheriff's office. Sheriff Grady Judd said his agency had been asked to interview Kazmierczak's father at the request of DeKalb County investigators, but said his agency would release no details.

David Heinzmann, Eric Zorn and Jeff Long write for the Chicago Tribune.