In 1999, Columbine High School, in Littleton, Colorado, faced at the time the largest school shooting in American history.

Now 20 years later, survivors still suffer debilitating side effects from the trauma of that day.

Four Columbine survivors spoke with Business Insider in 2018 about the aftermath of the tragedy, their efforts to help preempt future tragedies, and work with other survivors of mass shootings.

Visit BusinessInsider.com for more stories.

Editor's Note: This story was originally published in February 2018. It has been updated for the 20th anniversary of the Columbine shooting.

When a mass shooting grips a nation, as it did on Valentine's Day 2018, as reports of a shooting at a Florida high school unfurled, many individuals follow media coverage closely for updates.

For survivors of the Columbine High School massacre, the experience is different. Every new mass shooting has the ability to bring out unbearable trauma all over again.

"I have friends that filter [the news of a mass shooting] for me," Anne Marie Hochhalter, 37, told Business Insider in 2018. "They tell me info in texts. Seeing the images is very triggering for me and especially hearing sounds."

Family members embrace following a shooting at Marjory Stoneman Douglas High School, Wednesday, Feb. 14, 2018, in Parkland, Fla. AP Photo/Wilfredo Lee

Hochhalter was a junior in 1999, when two Columbine classmates stormed the school and took the lives of 12 students and one teacher before killing themselves. Hochhalter was shot in the back that day, paralyzing her.

"Physically, my whole life changed," Hochhalter said. "I had to relearn how to do everything. I had to cope with severe nerve pain, and then the depression with the loss of my legs. And then six months later, my mom took her own life. It wasn't because of Columbine, but it certainly didn't help her emotional state. My whole life was turned upside down."

The events at Columbine horrified viewers and changed the American psyche. Schools were no longer safe havens for children. But since the shooting in 1999, gun violence has only escalated. There have been 83 mass shootings in America since. Eleven of them were at schools.

Nearly 20 years later, Columbine survivors say some physical and emotional scars they bear will never disappear.

Read more: Americans look to Columbine to better understand school shootings — but myths around the shooters have persisted for years

For the victims, the day of the shooting is only the beginning

A makeshift memorial for the victims of the Las Vegas mass shooting. Getty Images/Drew Angerer

The 2017 Las Vegas shooting, which killed 58 people and injured 851, was a stark reminder for some Columbine survivors about how tenuous the nature of recovery is.

"Las Vegas was the catalyst for a lot of us going completely downhill," Hochhalter said. "I think it was because of that video footage and hearing the sounds and the screams — it took us all back. I've learned to turn myself off from it."

Heather Martin, now 38, was a senior at Columbine in 1999. She says she feels helpless when watching another school shooting unfold.

"As a survivor, that helpless feeling is amplified," Martin told Business Insider in 2018. "You know what they're in for and you know the event, while horrible and traumatic, is only the beginning."

Missy Mendo (right) and Heather Martin (second from right) at a survivor gathering in 2016. Shaun Martin

Martin didn't sustain physical injuries during Columbine, so initially she didn't realize how deeply set her trauma was. But she learned she was a changed person after a fire alarm went off in class during her freshman year of college.

"I started crying and I didn't even understand why I was crying," she said. "I didn't even know what the word 'trigger' meant at that point."

She later realized that the fire alarm was a trigger related to Columbine. She had been locked in a tiny room with 60 other students hiding that day, and the fire alarm was going off the entire time.

"Your brain will repress those memories and they are going to come out at some point in time in different ways," she said.

The 2007 mass shooting at Virginia Tech, which killed 32 people and wounded 17 other, brought on debilitating anxiety attacks for Martin. Then, in the wake of the 2012 Aurora theater shooting, which killed 12 people and injured 70 others, Martin took action.

She started a survivor's support group called The Rebels Project, named for Columbine's school mascot. The Rebels Project has about 600 members today, and has grown through word of mouth. But membership has also increased as more mass shootings happen every year.

I am terrified thinking about my baby's first day of school

Missy Mendo, 33, was a freshman at Columbine in 1999 and is a volunteer at The Rebels Project. "Each time something like this happens it has a different effect," Mendo told Business Insider about mass shootings. "The most recent one, seeing the images of the kids running out with their hands up was something that really resonated with me and really took me back," Mendo said. She described the Las Vegas shooting last summer as another "huge setback" emotionally.

The future also worries Mendo, who was six and a half months pregnant when she spoke to Business Insider in 2018. "I am terrified thinking about my baby's first day of school," she said.

Choosing forgiveness

Craig Scott, 36, was a sophomore at Columbine during the shooting. He was one of the students trapped in the school's library that day, and two of his friends were killed beside him. Scott's sister Rachel was the first student murdered that day.

Craig Scott's sister Rachel was the first murder victim at Columbine. Rachel's Challenge

"I had a lot of anger, even rage toward the shooters at my school," Scott told Business Insider in 2018. "I used to fantasize if I had just five minutes alone how I'd get revenge on them."

The anger came at a cost, Scott said. It was consuming him. On a trip to Africa a year later, he spoke to a man who lost 17 members of his family during the Rwandan genocide.

"He told me that forgiveness is like setting a prisoner free and finding out that prisoner is you," Scott said.

Choosing forgiveness was freeing, according to Scott. He and his family started a school assembly program in his sister's memory called Rachel's Challenge. Scott traverses the country, sharing Rachel's story and setting up kindness and compassion clubs and community events. The program reaches 3 million students a year, according to Scott.

'I have lost all faith in our government'

The No Notoriety Movement came about after the Aurora theater shooting. Reuters/Evan Semon

In the aftermath of a school shooting, gun reform is one of the most hotly debated topics.

But on this point, the four survivors of the Columbine shooting who spoke with Business Insider were less vocal than one might expect.

Martin and Mendo said their organization doesn't talk politics at all. They want the space to be a neutral place for all survivors of gun violence to join, regardless of political beliefs.

Scott, too, said his organization was apolitical, though on a personal level he was for "anything that helps make it hard for these kind of things to happen."

Anne Marie Hochhalter is a vocal supporter of the No Notoriety Movement, which urges individuals and the media to limit the use of the name and image of shooters after an event. The idea is that these shooters seek fame and glory, and by changing coverage in the aftermath of a shooting tragedy, the perpetrators will be disincentivized to act.

But their focus on the things they can control also seems to indicate a deeper disillusionment with government officials to affect meaningful change.

"I try not to get too political but I'm very passionate about gun control," Hochhalter said. "I have lost all faith in our government and senators and Congress people about making any lasting change because it has not happened and I don't think it's going to happen anytime soon."

She added: "I'd rather put my energy into something that can spur change."

'Look hard enough and you'll always find the light'

Students and young people led the "March for Our Lives" rally in D.C. on March 24, 2018. Aaron Bernstein/Reuters

As events unfolded at Marjory Stoneman Douglas High School in Parkland, Florida, Craig Scott watched some of the coverage, but not too much. Scott still finds it hard to fully tune in to footage of a mass shooting.

Instead he turned his thoughts to his mission of reaching individuals who have gone through a devastating event. He reminds himself and others that the aftermath of a tragedy may feel hopeless, but that feeling will eventually lessen. Pain can actually help push you toward a greater life purpose, Scott said. And although the events like Columbine and Parkland are horrific, there are glimmers of hope in the way the community comes together after.

"In these situations it feels like nothing but darkness, but there are lights within people in that community," Craig says. "There are heroes; there are stories of wonderful people who were killed. Focus on those lights and let them grow."