On April 20, 1999, two Columbine students, just three weeks shy of graduation, stormed the suburban Denver school armed with shotguns and semiautomatic weapons, fatally shooting 12 students and a teacher before committing suicide.
For the relatives of those they killed, April 20 evokes a mix of emotions from sorrow and anguish to fond memories of loved ones.
Betty Shoels, the aunt of murdered student Isaiah Shoels, said her 18-year-old nephew was a fun-loving athlete who was always smiling, despite feeling out of place as one of the school's few African-American students.
"What I miss most is his laugh," Shoels told Reuters. "He was just a great kid who loved to joke."
This year's remembrances were marred this week when a Florida teenager, who authorities said was "obsessed" with Columbine, traveled to Colorado where she died of an apparent self-inflicted gunshot.
Evan Todd was a sophomore at Columbine two decades ago when he was wounded in the school library, where 10 of the students were killed. He said whenever he hears of school shootings or other tragedies somehow linked to Columbine, it reminds him that he was "part of something so gruesome and so public."
He often recalls his football teammate Matt Kechter, who was shot dead just a few feet away from him.
"Sometimes I wonder what Matt would be doing now, what is life would be like," said Todd, 35, who is the father of a one-year-old son.
He credits his family and Christian faith for getting him through the months following the tragedy.
"I'm just thankful that I survived," he said.