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/ Source: TODAY
By Gabrielle Frank

This story is part of TODAY's series "Survivors: What Happens After the Headlines Fade." For more stories and videos from the series, click here.

It's hard to believe nearly 20 years have passed since the unimaginable tragedy that took place at Columbine High School in Colorado — one of the deadliest mass shooting attacks at a high school in U.S. history. Thirteen people were killed and 21 were wounded at the hands of two students.

Craig Scott remembers April 20, 1999 clearly. He was a sophomore at Columbine at the time, and that morning started like any other spring day. His older sister, Rachel, who was a senior, dropped him off at the entrance to the school so he wouldn't be late for class. That was the last time he would see her alive.

A few hours later, two students, 17-year-old Dylan Klebold and 18-year-old Eric Harris, went on a shooting rampage. Scott was able to escape. As soon as he ran out of the school, he sensed something had happened to his sister.

"I called my mom, and I said, 'Mom, I'm OK, but I think there's something wrong with Rachel,'" Scott remembered. The family waited all day and night for her to call. The following morning, authorities confirmed that Rachel was one of the first students killed.

Craig Scott (right) was able to escape Columbine High School on April 20, 1999, when two students went on a shooting rampage. His sister, Rachel (left), was killed.Craig Scott

The aftermath

Since that day, their quiet city of Littleton, Colorado, had changed forever.

The days and months following the massacre were challenging for Scott and his family. In addition to losing his sister, two of Scott's friends, Isaiah Shoels and Matthew Kechter, were murdered right next to him, while he played dead to avoid the wrath of the shooters.

"The days right after ... you're in so much shock," Scott said. "You could ask me a question, and I could just sit there, looking at you, because (I felt like) I'm not even there."

"I used to fantasize what it would be like to get revenge on the shooters," he recalled. "If I had just five minutes alone, what I'd do to them. And it started to eat me up."

The community came together and gathered around the Scotts and other families who had lost their loved ones.

"My family, we had people showing up to our home, bringing meals," he explained. He was also seeing professional counselors.

In addition, Scott felt support from the entire nation after appearing on TODAY for an interview with Katie Couric on April 22, two days after the massacre.

When Scott reflects on why he decided to go on the show, his answer is simple: "I wanted to give encouragement to Isaiah's dad," who appeared alongside him. "I wanted to give Isaiah's dad a hug."

Scott opened up to Couric about Rachel and the kind of warmhearted person she was. Eventually, spreading her message of kindness, compassion and acceptance would become his life's mission.

"It was the first time I started to talk about my sister on camera," Scott said. "I had no idea that would be something I would be doing a hundred, a thousand more times over the next 20 years."

Inspiring a chain reaction

When the Scott family began going through Rachel's possessions, they found an essay she had written about her ethics.

"She was talking about ... things like not judging or labeling other people, looking for the good in others and being a leader, showing mercy and forgiveness," Scott said, adding that at the end of the piece she wrote: "I have this theory that if one person will go out of their way to show compassion, it will start a chain reaction of the same."

Scott and his family found comfort in these words, and ever since, have used Rachel's message to inspire students around the world with their nonprofit organization, Rachel's Challenge. To date, they've shared Rachel's story with 25 million people.

"My family, in the months to come, started to feel a sense of purpose and mission with telling Rachel's story," Scott said. "And that — as we saw it — that was having an impact, that was part of our hope."

Craig Scott visits the Columbine Memorial, paying respects to his sister and the others killed in the 1999 shooting attack.

Scott gave his first high school presentation about Rachel's message when he was 18 years old. Since then, he's been working tirelessly to share her beliefs far and wide. While his dad, Darrell, is still overseeing Rachel's Challenge, Scott is now working with Value Up, a school assembly program focused on helping students realize their internal value.

"I think my story's a real eye-opener for some students going through a hard time," Scott said. "They can look at me and tell me ... 'I saw that you went through this, and look, you made something good out of it. I can get through what I'm going through.'"

Today, Scott's personal goal is to try to be a "big brother" or mentor to any student who has been through a school shooting. He hopes to help them figure out a way to turn their pain into purpose.

"I talk to them about forgiveness and letting go, and how if they don't, it can really imprison their heart," Scott said.

While it's difficult for him to open up about such a devastating time in his life, he believes that spreading his sister's message keeps her spirit alive.

"I think she would be proud of my family," Scott said. "I hope to see her one day again. ... If she were somehow aware of what has happened with her story and her legacy, and to say, 'Nice job, little bro,' that would be pretty cool."

Craig Scott continues to speak and work with schools throughout the country sharing lessons he learned from Columbine. Visit Value Up to learn more, or if you're interested in reaching out to Craig, please email him at craig@value-up.org.