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Spokesman Josh Rubenstein says the 12-year-old girl arrested in Thursday's shooting was being booked on a charge of negligent discharge of a firearm on school grounds.
Twelve-year-old Jordan Valenzuela, a classmate, tells The Associated Press that he talked to her just after the shooting.
He says she was sobbing and kept repeating, "I didn't mean it." He says she told him that the gun was in her backpack and that it accidentally went off when she dropped the bag.
The shooting left one teenager critically wounded and three other children injured.
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Sal Castro Middle School was placed on temporary lockdown as police searched the school as part of the shooting investigation.
Police officers swarmed the campus after the shooting was reported shortly before 9 a.m. Sal Castro Middle School is located on the campus of Belmont High School, Los Angeles School Police said. Approximately 50 LAFD firefighters and seven ambulances responded to the incident.
Investigators said the shooting happened inside a classroom. The school has been declared safe and there was no threat on campus.