Two school shooting threats in Los Angeles County, including one in which investigators say they found an AR-15 rifle, 100 rounds of ammunition, a list of intended targets and a drawing of a school's layout, were thwarted this week, authorities announced Friday.

In one of the incidents, a 13-year-old male student at Ánimo Mae Jemison Charter Middle School in Los Angeles was arrested after other students notified staff Thursday about an alleged plot, Los Angeles County Sheriff Alex Villanueva said at a news conference.

Officials added that the threat was heard in person and a search warrant was obtained for the suspect's home, where the weapon and ammunition were obtained. Villanueva said an investigation is ongoing to determine the origin of the rifle with a high capacity magazine, although it does have a serial number and was not a "ghost gun," a firearm that is assembled with different parts and has no serial number — a way for owners to avoid registering them.

Earlier, Los Angeles County Undersheriff Tim Murakami tweeted about the school shooting threat as well as a case in which the department's task force arrested a suspect for dealing and possessing illegal firearms, including a "ghost" rifle. He clarified in another tweet that the incidents are unrelated.

The school district involved in the school shooting investigation, the Green Dot Public Schools based in downtown Los Angeles, said its students are encouraged to speak up if they observe threats.

"We moved quickly to ensure that no harm came to our students," the district said. "Our students did everything right by raising concerns with adults."

Villanueva said authorities are grappling with a series of threats to schools in the wake of the Nov. 14 shooting at Saugus High School in Santa Clarita, California, a suburb north of Los Angeles.

The Los Angeles County Sheriff's Department said Thursday that the 16-year-old gunman in that case used a .45-caliber pistol — assembled from different parts into a "ghost gun" — to shoot five students, killing two of them. The teenager turned the weapon on himself and died the next day.

A motive for the shooting has not been determined, officials said.

In another incident on Thursday, the Los Angeles County Sheriff's Department said a shooting threat was made at a high school in Palmdale, about 60 miles north of Los Angeles.

A fight occurred on the campus of Pete Knight High School, after which a juvenile male involved "made threats on social media and posted pictures of a person holding a firearm," the sheriff's department said in a news release.

The male also admitted to writing threatening quotes and posting more threats online, authorities said. He was arrested for criminal threats and transported to a juvenile facility.