Days after Florida massacre, L.A. County authorities raced to thwart a school shooting plot in Whittier

Just 48 hours after a gunman slaughtered 17 students and teachers in Parkland, Fla., a security officer at a Los Angeles County high school overheard a troubling threat.

A student was plotting to create similar carnage at El Camino High School in Whittier, authorities said.

That information allowed...