Columbine High School

More than 200 doves flew over Clement Park on the day the Columbine community unveiled its memorial to the 12 students and one teacher who died during an attack at the Colorado school on April 20, 1999.

It had been almost a decade since the shooting, and the memorial was a circular expanse cut into a hillside next to the school. An outer wall formed the “Ring of Healing,” and an inner wall formed the “Ring of Remembrance.” Parents, classmates and teachers had submitted messages for the walls.

“The hardest part to understand,” wrote one student, “is kids killing kids.”

The memorial followed years of challenges.

In the days after the shooting, an Illinois man erected an initial tribute, placing 15 crosses atop a hill to recall the 13 victims and two student killers. A father of one of the dead students soon ripped out two of the crosses, and for months a debate raged about the appropriate way to recall the assailants.

A committee that had formed to create a permanent memorial eventually decided to avoid mention of the gunmen. It also agreed that the memorial should be separate from the scene of the attack.