With words of hope and healing, Americans marked the 10th anniversary of the Columbine High School massacre, in which two student gunmen killed 12 students and a teacher.
About 1,000 people gathered for a sunset memorial service at Clement Park, next to the school, where survivors, relatives and current students reflected on the massacre.
A dove was released for each of the 13 victims as principal Frank DeAngelis read their names.
Addressing the survivors, Mr DeAngelis said: "You were forced to grow up far too quickly."
Two seniors at Columbine unleashed an attack with guns and pipe bombs on the morning of April 20, 1999. A bigger bomb, which they hoped would destroy the crowded cafeteria, failed to go off.
The gunmen, Eric Harris, 18, and Dylan Klebold, 17, committed suicide.
"There are days I feel like it was yesterday. There are days it feels like a lifetime ago," said Val Schnurr, a Columbine alumnus who was wounded that day.
Bill Clinton, who was president at the time of the shootings, addressed the crowd in videotaped remarks.
"It's changed you, your community, your fellow Americans," Mr Clinton said of the tragedy.
The service drew hundreds of current students, many of them wearing Columbine's school colours, blue and white, and carrying flowers. Many said they scarcely remember the shootings.
"I feel like I owe it to the people who were hurt or killed, because it's just such a big part of our community," said Alyssa Reuter, 17, who was in second grade at a Littleton elementary school in 1999.
Flags flew at half-mast over the school in the south Denver suburbs, and mourners lay roses and carnations at the nearby memorial, situated on a hill overlooking the school. Many wiped away tears.
Columbine called off classes, as it has every year that the anniversary falls on a school day.
About 70 people gathered outside the state Capitol in Denver to push for gun control, while politicians inside passed a resolution honouring the victims.