It was not the first school shooting in the U.S. , but it grabbed the public’s focus and reverberates still: Twenty years ago this week, two seniors at Columbine High School in Colorado killed 12 fellow students and a teacher, wounded others … and turned the guns on themselves. The tragedy changed school security protocols and attitudes toward mental health … but similar scenes have played out since. The mother of one of the shooters, Sue Klebold, still wonders what she had missed … and has since tirelessly advocated for mental health awareness.