DeKalb, Ill. — The man who gunned down five students at Northern Illinois University on Thursday had recently stopped taking medication and had begun acting irrationally, according to people close to him, the authorities here said this morning.

“He was taking medication, had stopped, and had become somewhat erratic,” Donald Grady, the university’s police chief, said at a news conference.

Mr. Grady reiterated that there was no apparent relationship between the shooter, Steven P. Kazmierczak, 27, and any of the victims. And he said the police have no idea what the shooter’s motive might have been.

Mr. Kazmierczak, who was attending the University of Illinois at Urbana-Champaign, purchased two of the guns used in the shooting on February 9 at a local dealership, Mr. Grady said. On Thursday, Mr. Kazmierczak drove onto the Northern Illinois campus with those guns and two others. He then walked into Cole Hall with a shotgun in a guitar case and three handguns concealed in his coat.

Around 3 p.m., he kicked in a side door to the lecture hall where an ocean-science class was in progress and immediately began firing, Mr. Grady said. Police officers were on the scene by 3:03 p.m.

The university’s president, John G. Peters, praised the police response. The university had a response plan in place and did everything it could to respond quickly, he said.

“We were dealing with a disturbed individual who intended to do harm on this campus,” Mr. Peters said. “We did everything we could to ensure safety on this campus.”

Mr. Peters said he was now focused on helping the university heal. He kept repeating a single phrase: “We will get through this together.” —Libby Sander