As she dropped off her daughter at her first day of preschool in 2009, there were no tears for Colorado mum Amy Over.
Instead, she had a crippling panic attack.
“I couldn’t breathe. I almost collapsed to the ground,” Ms Over recalled.
After her husband rushed her to the hospital, medical staff asked her what she had been doing at the time of the attack.
“I said, ‘I just dropped off my daughter at school’,” she told The New Daily.
“I eventually learned the panic attack was a result of an underlying fear.”
It was a fear forged on one of America’s darkest days.
Ms Over is a survivor of the Columbine High School shooting in Littleton, Colorado on April 20, 1999 in which 12 students and one teacher were killed and a further 21 injured by seniors Eric Harris, 18, and Dylan Klebold, 17.
It was then the deadliest school shooting in US history.
Twenty years later, and with mass shootings part of the fabric of life in the United States, many of Columbine’s survivors are now parents whose children go to schools where “active-shooter drills” are routine.
“I feel like I let my kids go every day,” said Ms Over, who lives in Parker, Colorado with husband Curtis and their children Brie, 13, and Masen, 6.
“It was that feeling with Brie when I first dropped her off – how do I protect her? How do I keep her safe?”
On the day of the attack, Ms Over had been in a particularly good mood.
The night before, the 18-year-old received the news that she had won a basketball scholarship at a junior college.
And as soon as she arrived at school, the senior student went to see her basketball coach, Dave Sanders, who had helped with her application.
“He told me how proud he was of me and that to just go get my dream,” Ms Over, now 38, recalled.
“He had set everything up for me. I gave him a big hug. I didn’t know I was saying goodbye to him.”
At 11.19am, Ms Over was in the cafeteria when the shooters opened fire at students outside. Coach Sanders was also in the cafeteria.
“I heard the gunshots and coach Sanders told us where to go – to get under the tables,” Ms Over said.
“So I went underneath a table. And I heard the gunshots and their [pipe] bombs going off and I was trying to figure out where I was going to run.
“I was with a girl, Jamie. I remember telling her I couldn’t breathe, and she was like, ‘It’s OK. Everything’s going to be OK’.
“I ended up running out of the front door of the cafeteria. As I was running we were being shot at.”
She found refuge with others in a nearby home.
The 49-minute attack ended when Harris and Klebold turned their guns on themselves in the school library, where most of their murders took place.
Among their victims was coach Sanders, who had helped usher children to safety.
“The last time I saw him, he was running up the stairs,” Ms Over said.
In the aftermath, Ms Over struggled mentally and was forced to decline the junior college scholarship.
It took years and much therapy for her to work through her trauma.
By the time son Masen started kindergarten, Ms Over was a different person to the one who had collapsed at her daughter’s preschool in 2009.
“That was a big moment for me, just letting Masen go,” said Ms Over, who is the director of fundraising for The Rebels Project, a Denver-based support group for people who have survived mass shootings and other trauma.
“I get anxious sometimes, like if they have an active-shooter drill at school.”
Such drills involve doors and windows being locked and the students staying down.
“It’s necessary, but it’s sad,” Ms Over said.
On April 20, Columbine High School is holding a commemorative event, which Ms Over will attend.
“It’s going to be a hard day and just having my peers around – we all have a bond that will never be broken,” she said.
“I always reflect at every anniversary, but this year is monumental.
“And I get more emotional because of how far I’ve come. It hasn’t been the easiest road.”
To read more about The Rebels Project, click here.