Officials initially said they had the suspect, a female student, in custody; however, the student is now described as a person of interest. Mayor Eric Garcetti told local KABC-TV at the scene that it was unclear if the shooting was accidental.
The Los Angeles Times notes Castro Middle School sits just across the street from Belmont High School, in a building that "used to be part of Belmont High when the high school had a higher enrollment".
Two of the injured had "graze wounds", according to an official at Los Angeles County USC Medical Center, where four of the victims are being treated.
At a news conference outside the hospital on Thursday, Dr. Aaron Strumwasser said the 15-year-old boy was extremely lucky because the bullet wound failed to cause serious damage. "So I think he will do fine".
The Los Angeles Police Department (LAPD) confirmed to the BBC the female is 12 years old. The two schools share the campus. Many were crying as they were embraced by emotional parents.
Classes resumed as usual Friday, but counselors were on hand.
The district's interim superintendent, Vivian Ekchian, said that the school had been declared safe and offered thoughts and prayers to those who were hurt.
Shortly after that, however, the girl asked him to hide the weapon. The daily search is normally done at different hours of the school day. Parents who wished to pick up their children early were directed to a baseball field near the school at Loma Drive and Crown Hill Avenue.
Valenzuela believes the gun was in her backpack and that it went off when she dropped her bag. "As a parent, this is everyone's worst-case nightmare, a nightmare for all of us", Officer Arcos said.
The assailant was killed by police following the mass shooting in Rancho Tehama Reserve, northern California, which unfolded over several locations in the community, including the elementary school.
"We will attend to the needs of these students who witnessed this very carefully, with the understanding this is very traumatic", Zipperman said.
The school, which does random weapons checks of their students, does not know how she got the firearm to school or where she acquired it.
"I went insane. I couldn't handle it", he told ABC News.
A 12-year-old girl is in police custody following a high school shooting that left a teenage student fighting for his life.