Police at Ohio State University said Tuesday they were unable to determine how a janitor with a criminal past obtained two handguns used to fatally shoot a supervisor, injure another and then kill himself.

Nathaniel Brown, a janitor who had failed probation and blamed his supervisor for what he felt was an unfair job evaluation, opened fire in a maintenance building March 9 using a .45-caliber Glock pistol and a 9mm Luger.

In a final report, campus police said the weapons were traced to their last registered owners, including a retired Columbus police officer who told investigators that he believed he sold the Glock about five years ago to another officer but couldn't remember who it was.

The Luger was sold privately at a gun show in November 2009 and no records were kept.

Without a paper trail, investigators were unable to determine if the weapons were sold after that or how they ended up in Brown's possession, the report said.

Neither state nor federal law requires parties involved in private gun transactions to perform background checks or keep sales records, police said.

Brown, 50, spent about five years in prison in the 1970s and 1980s for receiving stolen property, court and prison documents show. He denied on a 2009 job application that he had been convicted of a crime, and an agency hired by Ohio State to run a background check on Brown did not turn up his criminal records.

Tuesday's 58-page report still leaves questions unanswered, including Brown's motive beyond an obvious dislike for supervisor Larry Wallington, who appears to have been the intended target in the attack.

Wallington was instrumental in Brown's pending dismissal, complaining that Brown was tardy and had problems following instructions.

The violence began when Brown quietly entered an office suite dressed in a trench coat and removed his guns, the report said.

His gunfire initially missed Wallington, who got up from his cubicle and ran. One bullet struck operations shift leader Henry Butler in the back. He survived.

Brown chased Wallington and gunned him down in a nearby garage bay, where Brown also died from a self-inflicted gunshot wound to the head, the report said.

"We don't know why he escalated this workplace conflict into a homicide. Only Brown knew that," said Ohio State Deputy Chief Richard Morman.

Evidence suggests that Brown, who was also fighting foreclosure on his home, planned the attack.

He sold his car the day before the shootings, telling the buyer that she could get him the money "whenever," the report said. Police also found letters in Brown's home addressed to a former girlfriend, telling her, "Sorry I let you down."

The report also uncovered that Brown, who had worked for 14 years as a night custodian at a Columbus auto dealership, was fired from that job for threatening physical harm to a co-worker who had walked on a freshly cleaned floor.

The shootings have led to proposed changes in Ohio State's employment policy.

The school announced earlier this month that civil service workers who don't pass probation will not be allowed to continue working on campus. The school also wants to streamline the way it conducts background checks on potential job candidates, choosing one outside vendor instead of five that are under contract.