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By Andrew Blankstein, Elizabeth Chuck and Associated Press

LOS ANGELES — A gunshot that went off in a Los Angeles middle school classroom on Thursday, hitting two students, was accidentally discharged from inside a girl's backpack, Los Angeles police said Friday.

A 12-year-old girl was booked on a charge of negligent discharge of a firearm after the shooting, police had said Thursday.

On Friday, LAPD Deputy Chief Robert Arcos told NBC News that evidence appeared to point to the weapon discharging once from within the bookbag — but he noted that investigators continue to examine evidence and interview witnesses.

The girl suspected of possessing the weapon was not identified due to being a juvenile. It's not clear why she had a gun in her backpack or how she got it. Arcos told NBC News the girl's parents are cooperating with investigators.

The gunfire erupted just before 9 a.m. PT at Salvador Castro Middle School, just west of downtown Los Angeles.

A 15-year-old boy who was shot in the head was in critical condition, and a 15-year-old girl who was shot in the arm was listed as stable, Los Angeles police said late Thursday. Los Angeles County-University of Southern California Medical Center said both were expected to make full recoveries.

Three others — a 30-year-old female staff member at the school and two other students, an 11-year-old boy and a 12-year-old girl — suffered superficial face and head injuries, police said.

Police recovered a semiautomatic handgun and described the shooting as an isolated incident involving negligence.

Jordan Valenzuela, a schoolmate of the girl in custody, told the Associated Press that she later said it was an accident.

The sobbing girl told him: "I didn't mean to. I had the gun in my backpack and I didn't know it was loaded and my backpack fell and the gun went off,'" the seventh-grader said.

Jordan, 15, later told the AP that the girl sat at the desk with her hands covering her face after the incident, and then she asked him to hide the backpack with the gun in it.

"I said 'No,'" Jordan told the AP. "Then I moved away from her because I was a little bit scared."

"She doesn't do bad things, she just stays quiet," he added.

Parents pick up their children at Salvadore Castro Middle School in Los Angeles on Thursday after two students were wounded by gunfire at the school.Frederic J. Brown / AFP - Getty Images

Steven Zipperman, chief of the Los Angeles School Police department, called the incident "very traumatic" for the kids in the classroom.

"One of the main missions we will have is the issue of finding out how a young person had access to a weapon," Zipperman said Thursday. "I assure you, if it came from an adult in a home, that the proper prosecutorial procedure will occur."

Andrew Blankstein reported from Los Angeles. Elizabeth Chuck reported from New York.