(CNN) Authorities arrested a 13-year-old boy and seized an assault-style rifle in connection with a reported threat at a middle school, the Los Angeles County Sheriff's Department said Friday.

Students at Ánimo Mae Jemison Charter Middle School told teachers and administrators on Thursday they heard the boy, a student at the school, threatening to shoot students and staff members, Los Angeles County Sheriff Alex Villanueva said at a news conference.

LASD Century station deputies responded to a call of a student who threatened to shoot other students and staff. The investigation resulted in a search warrant-the seizure of an AR15 rifle, ammunition, list of intended victims and a drawing of the school layout. Great job by Deps — Undersheriff Tim Murakami (@LASDMurakami) November 22, 2019

Deputies interviewed students and identified a suspect. During a search of the suspect's home, deputies seized an AR-15 assault-type rifle, about 100 rounds of ammunition, a hand-drawn map of the school and a list of names of students and staff members, said Sgt. Robert Dean.

"He did have a list of names and at this point we're trying to determine what that list was for," Dean said.

The firearm that was found is unregistered, but does have a serial number, Villanueva said.

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