DeKalb, Ill. ? If there is such a thing as a profile of a mass murderer, Steven Kazmierczak didn’t fit it: outstanding student, engaging, polite and industrious, with what looked like a bright future in the criminal justice field.

And yet on Thursday, the 27-year-old Kazmierczak, armed with three handguns and a brand-new pump-action shotgun he had carried onto campus in a guitar case, stepped from behind a screen on the stage of a lecture hall at Northern Illinois University and opened fire on a geology class. He killed five students before committing suicide.

University Police Chief Donald Grady said, without giving details, that Kazmierczak had become erratic in the past two weeks after he had stopped taking his medication. But that seemed to come as news to many of those who knew him, and the attack itself was positively baffling.

“We had no indications at all this would be the type of person that would engage in such activity,” Grady said. He described the gunman as a good student during his time at NIU, and by all accounts a “fairly normal” person.

Exactly what set Kazmierczak off – and why he picked his former university and that particular lecture hall – remained a mystery. Police said they found no suicide note.

Late Friday, a former employee at a Chicago psychiatric treatment center told The Associated Press that Kazmierczak was placed there after high school by his parents. She said he used to cut himself, and had resisted taking his medications.

Authorities also were searching for a woman who police believe may have been Kazmierczak’s girlfriend. According to a law enforcement official who spoke on condition of anonymity because the case is still under investigation, authorities were looking into whether Kazmierczak and the woman recently broke up.

Investigators learned that a week ago Kazmierczak walked into a Champaign gun store and picked up two guns: the Remington shotgun and a Glock 9mm handgun. He bought the two other handguns at the same shop: a Hi-Point .380 on Dec. 30 and a Sig Sauer on Aug. 6.

All four guns were bought legally from a federally licensed firearms dealer, said Thomas Ahern, a spokesman for the federal Bureau of Alcohol, Tobacco, Firearms and Explosives. At least one criminal background check was performed. Kazmierczak (pronounced kaz-MUR-chek) had no criminal record.

Kazmierczak had a State Police-issued FOID, or firearms owners identification card, which is required in Illinois to own a gun, authorities said. Such cards are rarely issued to those with recent mental health problems. The application asks: “In the past five years have you been a patient in any medical facility or part of any medical facility used primarily for the care or treatment of persons for mental illness?”

A Green Bay, Wis.-based Internet gun dealer who sold a weapon to the Virginia Tech shooter last year said he also sold handgun accessories to Kazmierczak.

“I’m still blown away by the coincidences,” Eric Thompson said. “I’m shaking. I can’t believe somebody would order from us again and do this.”

Thompson’s Web site, www.topglock.com, sold two empty 9 mm Glock magazines and a Glock holster to Kazmierczak on Feb. 4., though he had no idea whether they were used in Thursday’s rampage. Thompson said his site did not sell Kazmierczak any bullets or guns.

Kazmierczak, who went by Steve, graduated from NIU in 2007 and was a graduate student in sociology there before leaving last year and moving on to the graduate school of social work at the University of Illinois at Urbana-Champaign, 130 miles away.

Unlike Virginia Tech gunman Seung-Hui Cho – a sullen misfit who could barely look anyone in the eye, much less carry on a conversation – Kazmierczak appeared to fit in just fine.

Chris Larrison, an assistant professor of social work, said Kazmierczak did data entry for Larrison’s research grant on mental health clinics. Larrison was stunned by the shooting rampage, as was the gunman’s faculty adviser, professor Jan Carter-Black.

“He was engaging, motivated, responsible. I saw nothing to suggest that there was anything troubling about his behavior,” she said.