Two teenagers killed 12 students and a teacher at Columbine High School on April 20, 1999. The attack lingers in the popular imagination as the first in a string of modern-day school shootings in the United States.
The April 20, 1999 massacre of a teacher and 12 students shocked the world, marking the start of an era of school shootings in the United States and a fierce national debate about gun control that continues two decades later.
Survivors gave speeches at the ceremony, which also featured the presentation of a 17th-century Japanese tea bowl by Makoto Fujimura. The artist repaired the broken bowl by mending it together with gold.
"Being Columbine survivors, we found the strength and the courage to continue forging our path in life, but it didn't come without struggles," said survivor Mandy Cooke. "I do know that many of us continue to struggle because of that awful day."
Earlier in the day, visitors placed flowers at a memorial near the high school that features plaques dedicated to the victims.
Shooters' admirer stokes fears
The two Columbine shooters have inspired a small cult-like following since the attack.
Columbine was on heightened alert earlier in the week after police said they were searching for an 18-year-old woman who was "infatuated" with the 1999 shooting and had bought a gun in Denver.