LOS ANGELES (CBSLA) – An unregistered gun and ammunition were seized and a 13-year-old student was arrested for allegedly planning a mass shooting at his South Los Angeles school, authorities announced Friday.

The boy was taken into custody Thursday on criminal threat charges after planning a shooting at Animo Mae Jemison Charter Middle School in the Willowbrook area, the Los Angeles County Sheriff’s Department reported at a news conference Friday morning. An adult relative of the boy was also arrested on weapons-related charges.

“The threat alleged the student was going to shoot other students at the school on Friday,” Villanueva said.

Several students overheard the suspect making the threat and told their teachers, Villanueva disclosed. Teachers emailed L.A. Unified School District officials, who contacted the sheriff’s department.

L.A. County Sheriff’s deputies arrested the boy after conducting a raid at his home, where they seized an AR-15 rifle with a high-capacity magazine, about 100 rounds of ammunition, a drawing with a layout of the school and a list of intended victims.

“The subject also had in his possession a rudimentary hand-drawn map of the school, as well as a list containing names of both students and staff members from the school,” Villanueva said.

The gun was unregistered, but did have a serial number. The suspect’s identity were not released because of his age.

“The sheriff’s department acted on information provided by the school community about this incident,” LAUSD Superintendent Austin Beutner said in a statement Friday. “We all have a responsibility to keep students and all who work in our schools safe, and we are grateful to those who spoke up.”

This is the latest in a series of school threats in the wake of the Saugus High School shooting last week in Santa Clarita in which a student opened fire on his classmates – killing two of them and wounding three more – before turning the gun on himself.

“We’ve received a series of threats from the Saugus shooting forward to today,” Villanueva said Friday. “We’ve acted on all of them.”

On Thursday, deputies were notified of a fight which broke out at Pete Knight High School in Palmdale and resulted in several students being detained and suspended, Villanueva disclosed at Friday’s news conference. One of those students later posted a picture to social media of himself holding a gun and bullets.

After being interviewed and admitting to posting the photo, the boy was arrested on criminal threat charges, Villanueva said.

On Wednesday, a 15-year-old student was arrested for threatening to commit a shooting at Alta Loma High School in Rancho Cucamonga.

On Monday, a 17-year-old boy was arrested on suspicion of making criminal threats and illegal possession of ammunition after allegedly threatening Ramona High School in Riverside.

Also Monday, Whittier Christian High School in La Habra canceled classes over a purported threat that was determined not to be credible.