Columbine High School massacre
All students' backpacks were searched before classes began Thursday. At the time Hacienda La Puente Unified School District police told the "Pasadena Star-News" that the random checks were a precautionary measure related to the upcoming anniversary of the 
, which took place April 20, 1999. Eric Harris and Dylan Klebold killed 12 students and a teacher and wounded many others in Columbine High School in Colorado before committing suicide.
California student arrested in criminal threats made on Wikipedia