A 15-year-old boy was shot in the head is in critical but stable condition.
Jocelyn Lopez told KLTA that her younger sister, a 13-year-old seventh grader, was in the classroom where the shooting occurred.
The campus remained on lockdown officers cleared a second-story classroom at the middle school.
Two students, a boy and girl, were shot at Sal Castro Middle School in CA, according to police.
Another three people - aged between 11 and 30 - are reported to injured in peripheral circumstances, most likely in connection to attempting to escape the scene of the shooting. Two of the victims were 15-year-old students.
- Los Angeles Times (@latimes) February 1, 2018Update: One person detained following shooting on campus of Belmont High School/Sal Castro Middle School.
It is unclear how the 12-year-old shooter, who was taken to Los Angeles County's Central Juvenile Hall, was able to access a gun, police said.
In a telephone interview conducted alongside his mother, 12-year-old Jordan Valenzuela, who was in the classroom next door when the shooting took place, told the Associated Press that he spoke to the accused girl just after the incident.
"This child was extremely lucky", trauma surgeon Aaron Strumwasser said.
Television news footage shows someone being loaded into an ambulance.
In a morning news conference on Friday, police told gun owners that they must secure their weapons to keep them out of the hands of children.
Claudia Anzueto said her 12-year-old son was crying when he called her from a borrowed cellphone.
However, the police spokesman, Josh Rubenstein said that the authorities are believing that the girl didn't intend to open fire but the gun accidentally went off.
The U.S. Gun Violence Archive, an organization that tracks shootings in the U.S., said there were more than 2,000 documented unintentional shootings in the country in 2017 alone.