Police in Los Angeles said Friday they arrested a 13-year-old boy who allegedly threatened to shoot students at his middle school and seized an assault-style rifle with a high-capacity magazine during a search of his house.

When officers took the boy into custody, police said, they found a hand-drawn map of Ánimo Mae Jemison Charter Middle School in South Los Angeles and a list containing names of students and staff members.

Along with the boy, police arrested a 19-year-old male family member on a charge of possession of an unregistered firearm and confiscated about 100 rounds of ammunition, according to the Los Angeles County Sheriff’s Department.

The arrests are the latest effort by authorities to get ahead of a rash of school shooting threats that have rattled the Los Angeles area in the days since a 16-year-old student opened fire on his classmates at Saugus High School in Santa Clarita on Nov. 14, killing two and wounding three others. In the past week, police have arrested several teenagers on suspicion of threatening schools in the area and have investigated multiple reports of threats, some of which were found not to be credible.

At least one student who was arrested, a 17-year-old in Riverside, California, was found with ammunition, according to police. Another teenager was arrested after posting pictures of himself on social media posing with a gun and bullets and talking about a possible attack at a high school in Palmdale, California, police said.

“We’ve received a series of threats from the Saugus shooting forward to today, and we’ve acted on all of them,” Los Angeles County Sheriff Alex Villanueva said at a news conference Friday.

The 13-year-old’s alleged threats came to light when other students overheard him talking about gunning down his schoolmates, according to police. The students immediately told teachers, who notified the sheriff’s department.

“In this case, the fact that people stepped forward and said what they had heard led us to be able to prevent a tragedy today,” Villanueva said. “We have to have information to act.”

Police declined to name the boy or his 19-year-old relative, who is being held on $35,000 bail. Sheriff’s Sgt. Robert Boese told the Los Angeles Times the adult suspect will appear in court on Tuesday and likely face “a myriad of weapons-related charges.”

Interviews led officers to the boy’s house on Thursday. During their search, officers seized an AR-15 rifle – a weapon gunmen have used in many mass shootings – along with the high-capacity magazine and ammunition, according to Villanueva. The gun was unregistered, but it had a serial number, he said, differentiating it from the so-called ghost gun without a registration number used in the Saugus High School shooting.

“We’re trying to determine the origin of it,” Villanueva said.

The school, in the Willowbrook neighborhood of Los Angeles, is a college preparatory charter school where Los Angeles school police do not maintain a presence. It serves sixth- through eighth-grade students from the Watts and Compton communities of South Los Angeles, according to its website.

“The sheriff’s department acted on information provided by the school community about this incident,” Los Angeles Unified School District 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.”

At least seven shootings have taken place on American school grounds since the start of the academic year, according to a Washington Post analysis. The shooting earlier this month at Saugus High School was the first fatal shooting on a campus since the school year began. In the two decades since the shooting at Columbine High, some 233,000 schoolchildren have experienced gun violence at their own schools, according to The Post’s data.

Comments are not available on this story.