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(CNN) -- When she heard about Friday's mass killing at a Connecticut elementary school, Kacey Johnson felt her own trauma reawaken.
She is 1,800 miles away, and doesn't know anyone who was involved. But she knows something that very few other people do: what it's like when gunshots turn a school day into horror.
"It just sparks all my fears," says Johnson, a survivor of the 1999 massacre at Columbine High School in Littleton, Colorado.
Like many others who were a part of that tragedy, Johnson is now a parent herself -- and has a child in school.
"It brings up all this doubt in me that my child would ever be safe at school," she says. "Having been the one in a million that this happens to, my reality is different from most of the world. I don't have the thinking that this would never happen at my child's school. I think complete the opposite."
She and her husband Patrick have three daughters, ages 4, 3, and 6 months. The eldest goes to school. "I've had to come to a place of healing where I felt OK with sending my daughter to school, without putting any of my fears from my experience on her," Johnson says.
But after the news broke of the shooting at Sandy Hook Elementary School that left 26 people dead, 20 of them children, Johnson kept her daughter home Friday. "Because of all the emotion that the shooting brought up for me," she says, "and the battle in my mind."
That battle is between the thoughts that tell her kids will be safe and those that tell her that this could happen again, making one of her kids a victim.
It's a battle Michelle Wheeler also knows well. She was a senior at Columbine when two students killed 13 people before taking their own lives.
"I've witnessed more things than I'll ever want to admit to in my life," she says. "I've tasted blood. I've smelled blood. I've seen friends die. And to know that it happened in an elementary school -- they're just babies there. You're speechless. You don't know what to say."
She and her husband Michael have a daughter in first grade, who was sick at home on Friday. Wheeler is a pre-school teacher in an elementary school. She says she learned to move forward after Columbine by finding a "new normal."
"I am not your normal. I am my own normal. The memories will never go away."
Wheeler heard about the Connecticut shootings on Facebook, and is avoiding TV coverage. "I've looked at the news online, because as a survivor there's a control thing that I need," she says. "When Columbine happened, I didn't have control over my body. Your body shuts down. You go into fight or flight and it's really hard to turn your body back on. I need control in my life. I can control not turning on the TV, so I can control some of my emotions."
Sean Graves works every day to move past the horror of Columbine.
He lives with nerve pain. Graves was shot six times that day and spent more than a year in a wheelchair. Just last year he had surgery to try to correct some damage from a bullet that shattered his vertebrae and ricocheted out his hip.
"I go day to day without thinking about it. At least I try not to," he says. "But when something like this happens that's the first thing you do -- you begin to relive it... It'll take an emotional toll. That's for sure."
These tragedies "can completely trigger some of the nightmares and flashbacks," damaging recovery and bringing back feelings of terror and powerlessness.
If people start to have trouble functioning or feel very anxious and fearful, they should consider resuming therapy, Bloom says.
Avoiding the news coverage, or seeing just a little, is often a good idea in those cases, she says.
"People need to be cautious," Bloom says.
But depending where someone is in his or her recovery, there can be another response to a tragedy, Bloom says. Survivors may feel a sense of strength to help shattered families because of their ability to empathize.
"You can be inoculated, welled up with compassion and less of the fear because you know you can get through to the other end, because you've been there. People who have been through this have found a way to create meaning," she says. "People take their experience and transform their pain into power. It's psychological alchemy."
That's something Columbine survivors are working to achieve. Their thoughts and prayers are with the families in Connecticut.
"It's taken me years to get to the point where I'm at now," says Graves. "It's been a struggle, and it's been painful and emotional. But to see a bunch of families have to go through this -- they're just starting their journey now. I wouldn't wish this on my worst enemy."
Wheeler and some other survivors and their parents have created a group called The Rebels Project to reach out and help survivors of mass shootings and families of victims.
"We have parents who had to carry us through dark ages and try to get us to be normal citizens," she says. "These parents know how to speak to parents. They are working to do something for Connecticut."
The project was started this past summer, after a gunman opened fire at a movie theater in Aurora, Colorado, killing 12 people.
Some people in the Aurora community who were traumatized by that mass killing have already been calling a hotline in the city, says Mara Kailin, program director at the Aurora Mental Health Center.
"We've been hearing from from first responders and folks in the community that were directly impacted," she says. "They're wanting to help people in Connecticut -- and being reminded of what we went through a very short time ago."