Investigators said Friday that they know what motivated one high school student in Knoxville, Tenn., to fatally shoot another.

Darrell DeBusk said that the killing of 15-year-old Ryan McDonald on Thursday had "nothing to do with being gang-related or racially motivated."

He said police won't say specifically what the motive was, but said it was based on a brief statement that 15-year-old suspect Jamar Siler made to a police officer.

DeBusk said the motive won't be revealed until a hearing scheduled for Aug. 28.

Classes resumed at Central High School on Friday morning with a heavy police presence.

The shooting happened in the school's cafeteria before classes started Thursday. Police said the suspected shooter was taken into custody minutes later on a nearby street.

The suspect and victim knew each other, Knox County School System Superintendent Bill McIntyre said.

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"This wasn't a shooting that was a random act," Chief Deputy Bill Roehl said. "It was an individual directing his aggression toward another individual, not the school or the students inside the school."

The cafeteria was a popular place to gather before classes started at 8:30 a.m., students said. Chad Griffin, 15, and Josh Matthews, 14, said that they were sitting about 10 feet away from the victim and talking when they heard a sharp noise.

Griffin at first thought someone had dropped a book and then looked around.

"He got shot and started walking and he was holding his chest. There was blood everywhere. And then he fell and his arm hit me," Griffin said.

Matthews said he thought it was a fake at first but then realized the shooting was real.

"I took off running and ran outside and called my mom," Matthews said.

Students in the cafeteria began crying and scrambling to leave, while others tried to get in the room, thinking they had missed a fight, witnesses said. Students began to gather around the victim, said freshman Jared Wohlford, 14. "Everybody started running out real fast saying, 'He got shot,"' he said.

The school, which has about 1,400 students, was placed on lockdown after the shooting. Classes were dismissed and students were bused to a nearby church so they could be picked up by their parents.

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The Associated Press contributed to this report.