Los Angeles County authorities said Friday they thwarted a potential mass shooting after seizing an AR-15 rifle, ammunition, a list of intended targets and a drawing of a school's layout during an arrest.

Details about the case were not immediately available, but Los Angeles County Undersheriff Tim Murakami tweeted that the discovery was made in response to a call about a student who wanted to shoot other students and staff. He added that the suspect, who was not immediately identified, purchased parts on the internet to build a "ghost gun" — meaning it was assembled at home and had no serial numbers, a way for owners to avoid registering them.

Murakami also said silencers and body armor were located in the investigation.

The school district involved in the investigation, the Green Dot Public Schools based in downtown Los Angeles, said in a statement that its staff was notified Thursday about a possible threat.

"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."

The district operates 20 public schools in the Los Angeles area, but did not say which school appeared to be the target.

Discovery of an alleged threat comes a week after 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 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.

This is a developing story, please check back for updates.