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LOS ANGELES — A 13-year-old boy was arrested for allegedly threatening to shoot other students and staff at Animo Mae Jemison Charter Middle School in the Willowbrook area, and sheriff’s officials said Friday he had an unregistered AR-15 rifle and 100 rounds of ammunition in his possession.

The unidentified student allegedly made threatening remarks on campus Thursday, suggesting a shooting would be carried out Friday, Los Angeles County Sheriff Alex Villanueva said.

“Multiple students overheard the school threat on campus and teachers emailed the administration about concerns raised by the students,” Villanueva said. “School officials contacted the sheriff’s department immediately.”

After identifying the student, detectives served a search warrant at his family’s home and found a “rifle with a high-capacity magazine” and a “rudimentary hand-drawn map of the school as well as a list containing names of both students and staff members of the school.”

Undersheriff Tim Murakami said on his Twitter page the weapon was an AR-15 rifle, and he characterized the list of students and staffers as a “list of intended victims.”

Villanueva said the weapon did have a serial number, but it was not registered. Its origin was under investigation.

“With all incidents like these, we ask all Los Angeles County residents that if they see or hear something to say something,” Villanueva said. “In this case, the fact that people stepped forward to say what they had heard led us to be able to prevent a tragedy today.”

The student was arrested on suspicion of making criminal threats. Sheriff’s officials said a 19-year-old man, who is a relative of the student, was also arrested on suspicion of possessing an unregistered firearm. He was being held in lieu of $35,000 bail, but his name was not immediately released.

Green Dot Public Schools, which operates the charter campus, issued a statement saying it has “processes in place that allow our students to say something if they observe threats or have safety concerns.”

“Yesterday, our staff was notified by students of a threat, and we moved quickly to ensure that no harm came to our students,” according to Green Dot. “Our students did everything right by raising concerns with adults. We are in close contact with law enforcement and have been informed that there is no longer a safety concern. We are deeply thankful for the partnership and quick response from law enforcement around this incident.”

News of the arrest came eight days after a 16-year-old student took a gun to Saugus High School and shot five other students, two fatally, before shooting himself in the head. That gunman died at a hospital the next day.

There have a been a rash of threats targeting various campuses over the past week. Covina police beefed up staffing Friday at Charter Oak High School in response to a possible threat targeting the school. Sheriff’s deputies in Palmdale, meanwhile, said they arrested a student late Thursday for allegedly posting online threats targeting Pete Knight High School.

Late Friday morning, Pasadena police went to La Salle High School after a smartphone notification app mistakenly circulated unfounded information of an active shooter on campus, said Lisa Derderian of the city of Pasadena. Officers searched the campus but found no shooter, Derderian said.