Sheriff’s deputies in the Los Angeles area found an AR-15-style rifle, 100 rounds of ammunition, a list of names and a drawing of a middle school in the home of a 13-year-old boy who classmates heard threaten to carry out a school shooting, authorities said.

The 13-year-old was arrested on suspicion of making criminal threats, according to Sgt Bob Boese, a spokesman for the Los Angeles county sheriff. A 19-year-old male relative also was arrested on suspicion of possessing an unregistered firearm.

At least 30 threats against schools in the Los Angeles area have been investigated in the week since a shooting at Saugus high school in Santa Clarita, where a 16-year-old shot dead two fellow students and killed himself.

Only two of those threats have been deemed credible and resulted in arrests, Boese said.

In the first incident, deputies searched the 13-year-old’s home after multiple students at Ánimo Mae Jemison charter middle school in Willowbrook, just south of downtown Los Angeles, overheard him say he would carry out the shooting on campus the following day, said Alex Villanueva, Los Angeles county sheriff.

The students alerted teachers and police were notified.

Villanueva praised school officials for quickly alerting authorities and said the fact that people shared what they heard with law enforcement had prevented a tragedy.

In a second case deemed a credible threat, in Palmdale, north of Los Angeles, a student at Knight high school was arrested after he made threats on social media following a campus fight, officials said.

The sheriff said the boy acknowledged posting threats along with pictures of a teen with a gun. No weapon was recovered.

Several other threats of violence against schools in Santa Clarita, where the Saugus shooting occurred, were investigated and found not credible, CBS Los Angeles reported.