"The anniversary is always difficult," he said.
He used to watch his son Daniel play there, close to the high school where he was later killed on April 20, 1999.
After the Columbine High School shootings, Mauser was given the shoes his son was wearing that day. It was then he realized they wore the same size. From then on, he would wear those shoes when he would make public appearances. Now, he's written a self-published book called "Walking in Daniel's Shoes."
Mauser says a meeting in 2009 finally prompted him to write down what he had thought and felt for so long. He was able to sit face-to-face with the parents of the boys who killed his son.
"Some of the edge was taken off because I had written to both sets of parents beforehand and I really unloaded there," Mauser said. "We spoke to them parent-to-parent. We wanted an understanding of who were these boys."
"While I think there could have been more intervention into their son's life, I am also convinced that this could happen to an awful lot of young people and indeed that is reflected in some of the messages that I get," Mauser said.
Mauser has become known for his stance on gun control since the shootings.
"I didn't have an idea of what I was getting into, frankly," he said.
In his book, he writes about the criticism he's gotten, such as accusations that he's politicized his son's death. He has received death threats.
He says his reasons for speaking out are misunderstood. He says his reason is Daniel.
"He joined the debate team even though he was shy in front of other people. He wasn't at all athletic, but he joined the cross country team. So he took on his weaknesses, he didn't run away from them," Mauser said.
He says that's what got him into politics and got him to champion a cause.
"For me, it is really standing in Daniel's place on the debate team doing what I think he would want me to do," he said.
Mauser expects he'll lose money on his self-published book, maybe break even, but that isn't the point. He says it part of the growing memorial in his heart to Daniel and to everyone who suffered.
"You always remember that there were 13 who were murdered that day and the special lives that were lost. We shouldn't lose sight that there are a lot of other amazing teachers and kids out there and we should be doing everything in our power to protect them from something like this," he said.
Mauser got the first copy of the book on Thursday.
"I think I most want people to understand what it is like to be in the middle of an event like this," he said. "What it is like to be grieving. What it is like to be in the middle of this media frenzy, the controversies, to give a people a feel of what that was like."
Mauser says he still gets Internet messages from those who idolize the killers.
"[They] think nothing of saying vile things to someone who has lost their son. Like, 'Your son deserved to die,' and, 'Long live the killers,'" he said.
In the book he wrote, "If we fail to do so something with these troubled, bullied, disaffected kids, we're going to continue to suffer tragedy."
"The most important thing we can learn from this is that it can happen to anybody anywhere in this country," Mauser said.
To learn more about Mauser's book, visit www.danielmauser.com.
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