LA County Sheriff’s patrol cars leave El Camino High School in Whittier, Calif., on Wednesday. A 17-year-old student threatened to open fire at the school days after the massacre at a Florida school. An alert security guard who overheard him reported it to police, who went to the boy’s home and found two AR-15 rifles, authorities said Wednesday.

LA County Sheriff’s patrol cars leave El Camino High School in Whittier, Calif., on Wednesday. A 17-year-old student threatened to open fire at the school days after the massacre at a Florida school. An alert security guard who overheard him reported it to police, who went to the boy’s home and found two AR-15 rifles, authorities said Wednesda