At 11:21 a.m. on April 20, 1999, the first 911 call alerted authorities to the unfathomable: Two students at Columbine High School, 18-year-old Eric Harris and 17-year-old Dylan Klebold, had launched what was then the deadliest school shooting in U.S. history.

With shots fired outside the school, they began a spree that left a dozen classmates and a teacher dead, and many more wounded, before the two committed suicide in the school library. A chilling collection of writings and videos would reveal the darkest side of disaffected youth and a grandiose plan to use an arsenal of guns, pipe bombs and larger explosives to kill and maim.

Ten years later, The Denver Post looks at the legacy of Columbine and visits the Class of '99 and the principal who remains at the school to this day.