LITTLETON, CO (CNN) – Sunday marks 15 years since one of the deadliest mass shootings in American history.
On April 20, 1999, two students attacked classmates and staff at Columbine High School.
The massacre killed 12 students and a teacher. Both teen gunmen then killed themselves.
The campus library – where most of the killings took place – was later demolished and replaced with a student memorial.
A larger memorial was built adjacent to the campus and opened to the public in 2007.