A police officer accidentally fired a live round during active shooter training this week at a Tennessee middle school, officials said.

No one was near the officer Thursday morning, and the bullet damaged a "minor little piece" of a cinderblock wall at Rockwood Middle School, Roane County Schools Superintendent Leah Watkins told WBIR-TV (http://on.wbir.com/2aSdMjD).

Police and the school system were working together to demonstrate what an active shooter situation sounds like. In the drill, school employees are locked in a classroom with the police chief while an officer fires blank rounds in the hallway.

"It was a blank-fire drill, is what it was," Watkins said. "There was not to be any discharge of live ammunition."

The incident is being considered an accident, but police aren't sure how a live round came to be fired instead of a blank and are investigating.

Rockwood Police Chief Danny Wright said it illustrates a lesson: "No matter how often you handle a weapon or how familiar you may be, you always treat a weapon like it's loaded."

Police successfully conducted the same drill at Rockwood High School on Monday. Another drill planned later Thursday for Ridge View Elementary School was canceled.

"We just felt it was in the best interest to discontinue that simulation," Watkins said.

The officer who fired the weapon is on paid administrative leave, which is standard procedure while the incident is investigated.

The incident happened two days after a Florida police officer shot and killed a woman during a citizens academy roleplaying scenario. The Florida officer's gun was supposed to contain blanks.

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Information from: WBIR-TV, http://www.wbir.com/