TOM Mauser remembers brushing off his 15-year-old son Daniel when he started asking about loopholes in gun laws.
Just two weeks later Daniel was dead - shot and killed at Columbine High School by a gun bought through the very same loophole he was asking his dad about.
Tom knew there was only one way to honour Daniel's memory.
Since the massacre on April 20, 1999 - in which 12 students and one teacher were slaughtered in an indiscriminate shooting spree - he has dedicated himself to campaigning for more stringent gun control laws both in his home state of Colorado and across the United States.
Recalling the poignant conversation, Tom told Sun Online: “Daniel was a member of the debate team at Columbine and he heard something in conversation with kids in his class and that night he asked me at the dinner table: ‘Dad, did you know there were loopholes in the Brady bill?
"The Brady bill was the national law that requires licensed dealers to have a background check conducted for anybody who wants to purchase a gun.
"I just kind of blew it off and told him, ‘No, I didn't know about that Daniel’. I knew what the Brady bill was, but I didn't know there were loopholes in it. I didn't really follow the issue.
'HOW COULD I NOT CAMPAIGN FOR GUN CONTROL'
“Two weeks later Daniel was killed with a gun that was purchased through a loophole in the Brady Bill.
“How could I not act on that? How could I not see that as a sign of what I was supposed to do?
"So that's how I honour him is by continually following his words, his question to me, following up on it and campaigning for change."
Sadly despite 20 years of campaigning, little has changed in terms of gun control in the States.
Since the horrors of Columbine, an estimated 223,000 children have been exposed to gun violence at school, according to recent figures.
"Nothing has changed," Tom said. "If anything we've regressed."
"We've made it much easier for the gun industry to do what they do - we've given them immunity from lawsuits.
"At the state level, it's been different. Here in Colorado we have limited magazine capacity, and we require universal background checks.
"That started in 2000 - the year after Columbine - when we closed the gun show loophole, in which people buying from a private seller didn't need a background check.
"Three of the four guns used at Columbine were purchased at a gun show, through the gun show loophole. And other states have been doing that type of thing.
"But while the progressive states are passing stronger gun laws - in the other states they're going the other direction, they're making it easier to carry guns.
"More places are arming teachers, they're allowing guns to be carried in bars, churches and all kinds of places.
'CAN WE NOT AT LEAST REDUCE THE BODY COUNT'
"So we just have this big divide just like we do politically and at the national level, nothing is done.
"It's going to take one heck of a big change in Washington to make anything happen at a national level.
"I understand it's unrealistic to ban guns - and I've never campaigned for that. But can we not make some changes? Can we not at least reduce the body count?"
Tom still gets emotional when he thinks about his son - he says he can't bear to think about all the milestones the youngster missed; graduating, getting married, having a family.
Instead he remembers Daniel as a 15-year-old boy who loved video games and playing with his sister.
“My memories of Daniel, are sadly fading after 20 years, and that breaks my heart," Tom said.
"He was a very quiet boy, who loved to play video games.
“A very playful kid with his younger sister. He was a boy scout and a very intelligent kid.
"He’d become a straight A student, a brilliant kid. But a kid who took on his weaknesses.
“He was not athletic and yet he joined the cross country team at Columbine. Even though he never made the squad he would always practice with them.
"And he was so shy, the last kid you'd expect to stand up in front of other people and talk but he chose to join the debate team at Columbine. So I just admire him for taking on those weaknesses and facing them."
'I CAN'T FORGIVE THEM FOR KILLING'
Tom admits he still struggles with his anger towards Dylan Klebold and Eric Harris, the two twisted teens who carried out the atrocity.
“When it comes to the shooters I think they were two very mentally disturbed kids," he said.
"My anger is that they didn't have the courage to deal with their problems the way the Daniel dealt with his problems and his weaknesses.
“There was a lot of talk about them being bullied and they were getting back at the bullies but that's bullsh*t, it's just nonsense.
“Yes I think they were bullied but they also were bullies themselves.
“I can forgive them for being two very lost and mentally disturbed kids. I can't forgive them for killing. I just can't bring myself to do that.
"I can't forgive them for murdering - because my son didn't deserve it. None of these victims deserved it.
“So I’ve reconciled myself to that."
Tom also met with the parents of both shooters in a bid to gain closure - which he said helped somewhat.
"I certainly hold them a little responsible for not doing more to have prevented it, to have not been more tuned into what the hell was happening with their kids," Tom said.
“Eventually after eight or nine years of nothing being said by Eric Harris’s parents they agreed to meet me.
“I came to the conclusion they were somewhat clueless people.
"They definitely carried a lot of guilt around with them. Their lives were turned upside down.
"And I don’t lose sight of the fact that they lost a kid too and they will have to bear this burden for the rest of their lives that their kids killed other kids. I don’t have that burden to bear - they do.”
While the Mausers still struggle with grief over the loss of their beloved son, both Tom and wife Linda were determined to do everything they could to honour his memory.
They even adopted a baby from China in Daniel's memory, who is now 19-years-old.
"She means a lot to us," Tom said. "She kind gave us a place to put our energy instead of just negative energy.
"We still had another daughter to raise, but she was very supportive of the adoption. It helped us an awful lot.
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"We've shared lots of information with her about the brother she never knew."
Meanwhile Columbine survivor Craig Scott told Sun Online how he's helped to stop 12 other school shootings by visiting schools across the country.
And mum-of-four Kacey Ruegsegger Johnson revealed how she still lives with daily pain from her injuries after she was shot at close range by the shooters and miraculously survived.