The school shooting left a 15-year old boy in critical condition with a gunshot wound to his head; a 15-year-old girl in fair condition with a gunshot wound to her wrist; a 30-year-old woman who was not hospitalized with an undisclosed injury; and two other students, aged 11 and 12, with minor injuries after being grazed.
The girl was taken into custody without incident after she and witnesses were interviewed, officials said.
Steven Zipperman, chief of the Los Angeles Unified School Police Department, said a weapon had been recovered.
"I went insane. I couldn't handle it", he told ABC News.
Sal Castro Middle School is close to the city's Westlake area, approximately two or three miles northwest of downtown, and it imparts a grounds to Belmont High School. Lt. Chris Ramirez with the LAPD said negligent can mean many things, but most likely a "mishandling".
Jordan tells The Associated Press that his 12-year-old classmate at Salvador B. Castro Middle School told him it was an accident.More news: Klopp refutes Van Dijk weight-loss claims
The school has about 365 students in grades 6 through 8 and most are Hispanic and many from low-income families. Over 360 students are enrolled in the middle school, where weapon checks are conducted regularly. Some schools failed to do the searches daily, the audit found. One-fourth lacked enough metal-detecting wands to search properly.
After the suspect was taken into custody and the wounded students were taken to hospitals, police went room to room on the locked-down campus, leading some students out with their hands behind their backs.
Anxious relatives wait outside Sal Castro Middle School after a shooting on February 1, 2018.
"They thought it was a toy gun, but then it shot", KABC reported, quoting Benjamin Urbina.
A day earlier, a 16-year-old student was apprehended after wounding a student at a Texas school.
At least a dozen school shootings have occurred already in 2018. McNair was among many parents who rushed to the scene after seeing reports of the shooting on television.More news: Fox gets Thursday night football for 5 years
At a school event last month where good attendance certificates were presented, principal Erick Mitchell said the campus is becoming a destination for families who want a smaller school setting, the Los Angeles Times reported.
"Mom I'm scared", the first one said.
She said her sister was fine and had been texting with her.
In another troubling incident in 2016, two teens at a high school in Arizona died from the same bullet, in an apparent murder suicide.
The district started random searches in 1993 after a 16-year-old was shot and killed at Fairfax High School.
"We have a lot of people from the mayor's Crisis Response Team as well as counselors from the school district to help these young people deal with the situation to make sure there's not further trauma to them from what happened", he said. "It's the best-kept secret in town".More news: Airline CEO takes pay cut to support gender equality