Police policy was not followed leading up to a school assembly where a student pulled the trigger of an assault rifle mounted on a police motorcycle and three children were injured by bullet fragments, authorities said.
Department rules state the rifle chamber should be empty when it is mounted, Police Chief Miles Pruitt said in a statement late Thursday.
"The preliminary investigation has shown that the established safeguards and procedures were not followed," Pruitt said. "It is unfortunate that this event occurred and I will make every effort to ensure it does not happen again."
The AR-15 was locked to the side of the motorcycle that was on display at Newman Elementary School on Wednesday during an anti-drug program when a student managed to fire it, Chino police spokeswoman Tamrin Olden said.
"The bullet disintegrated when discharged by striking a metal plate where the barrel rests against the weapons mount," and fragments apparently struck some children and caused minor injuries, she said.
Olden said that the children were treated at a hospital and released.
A third student was examined at the school and released to parents, she said.
The weapons mount holding the rifle had several fail-safes, including the metal plate, Olden said.
There was still no word late Thursday on whether a police officer was nearby when the discharge occurred.
Rifles have been removed from other police motorcycles while the shooting is under investigation, Olden said.
The shooting occurred while police were visiting the school for Red Ribbon Week, an annual national event that features a series of activities designed to raise awareness about drug and alcohol abuse.