The columbine is a perennial flower often seen growing wild in the Rocky Mountains. But since April 20, 1999, the word has been a universally recognized reference to what was then the worst mass shooting at a school in American history. Thirteen people were shot to death and 21 were wounded by a pair of Columbine High School students who then killed themselves. The massacre in Littleton, Colo., was a singularly shocking event.