Several hundred people attended the ceremony in Littleton, The Denver Post reported.
Planning for the memorial began in 1999, not long after two Columbine High School students, Dylan Klebold and Eric Harris, killed 13 people and wounded several others before taking their own lives. Fundraising problems caused the memorial to be scaled down, with the final cost coming in at $1.5 million.
The memorial includes a tablet for each of the victims with inscriptions chosen by their families.
Littleton Fire Chief Jay Ruoff, who responded to the shootings, was at the ceremony. He remembered the “chaos” at the time as emergency workers and parents tried to figure out what was going on inside the school.
"Today is about reflecting on all the kids that were in the tragedy, senseless tragedy," he told the Post. "It's behind me, but I still think about the kids."