CHICAGO – As hard as he tried, Jim Thomas on Friday still couldn’t fathom how the personable, fun-loving man he called his friend and the man who shot five Northern Illinois University students to death could be the same person.

The Steven Kazmierczak he knew helped students with their homework and helped friends move into new homes.

“Steve was the most gentle, quiet guy in the world. … He had a passion for helping people,” said Thomas, an emeritus professor of sociology and criminology at NIU. “He was such a fun guy, with an incredible sense of humor – an intellectual.”

But police say the 27-year-old Kazmierczak, armed with handguns and a pump-action shotgun he carried onto campus in a guitar case, stepped onto the stage of a lecture hall Thursday and opened fire on a geology class. He then committed suicide.

“I can’t reconcile those two images,” Thomas told The Associated Press in a telephone interview. “Steve was a friend and he did a horrible, horrible thing.”

The 66-year-old came to know Kazmierczak when he took one of Thomas’ introductory sociology courses around 2004. The class was taught in Cole Hall – the site of Thursday’s deadly shooting.

“He was one of the brightest students,” said Thomas. So much so that Kazmierczak was soon drafted as a teacher’s aide.

Over the years, the two went from having a student-teacher relationship to becoming friends.

“I was probably closer to him than any faculty person,” said Thomas, who also co-authored a 2006 academic paper on self-injury in prisons with Kazmierczak.

One thing that, for some, might raise a red flag in retrospect was what Kazmierczak once told Thomas about getting a psychological discharge from the military. But Thomas said he doesn’t think that necessarily sheds light on the shooting.

“It was no major deal, a kind of incompatibility discharge – for a state of mind not for any behavior,” he said. “He was concerned that that on his record might be a stigma.”

Kazmierczak talked about working with kids, possibly in the criminal justice system; he also contemplated getting a Ph.D. and pursuing a full-time career in academia, Thomas said.

“He was very studious,” he said, “almost a perfectionist.”

Thomas, a Grand Rapids, Mich. native who moved to teach in DeKalb nearly 30 years ago, said he never saw anything in Kazmierczak’s personality that made him question his self-control, describing him as warm and socially at-ease.

“Even when he got a little miffed, it was strange because he almost never got miffed,” he said. “Every time I saw him he was with people. And he had a dynamite female friend. … They got along well. It was never abusive.”

Thomas declined to discuss some details about Kazmierczak, including whether he might have been taking medication.

Since the killings, Thomas has tried to envision Kazmierczak walking into the hall, taking out a gun and pulling the trigger.

“I try to picture him, and I can’t picture him being angry,” he said. “So I see him walking in and being very businesslike, not going in rage – maybe task-oriented. But when I do that, I get disconnect. … It just doesn’t come together.”

Thomas joked for a moment about how he’ll try to come to terms with what his friend did, saying, “I’ll just get a good bottle of Bourbon and go into denial.”

But even if he wanted to avoid it, Thomas will have little choice but to confront the questions again about how his friend could be capable of such brutality: On Monday, Thomas said he was scheduled to teach a class in a building next to Cole Hall.