AMNA NAWAZ: It's been 25 years since 12 students and one teacher were killed in the Columbine massacre in Littleton, Colorado.

It was the largest mass killing at a high school in U.S. history at the time.

Since then, overall school shootings have grown to much higher levels.

And there have been several mass shootings with tragic losses at schools like Sandy Hook, Parkland and Uvalde.

Tom Mauser lost his son Daniel at Columbine that day.

And he joins me now.

Tom, welcome to the "NewsHour."

Thank you for joining us.

TOM MAUSER, Father of Columbine Victim: Thank you.

AMNA NAWAZ: So, 25 years, when you say, it can feel like a very long time and maybe not that much time at all.

I hope you don't mind if I just begin by asking, how are you doing?

TOM MAUSER: You know, doing well.

There's healing.

Some healing comes with time.

It doesn't seem like 25 years to me, frankly.

AMNA NAWAZ: Why is that?

TOM MAUSER: I really can't explain it.

I mean, for one thing, to me, Daniel is still 15.

I don't think of him, what he would be doing today or what he'd be like.

He's still 15.

AMNA NAWAZ: And you have dedicated your life and your days to honoring Daniel, to doing what you can to end gun violence.

You have been traveling around the country and talking to people.

Tell me a little bit about that work.

What's that journey been like and what have you learned along the way.

TOM MAUSER: Well, I have met a number of other people who have suffered this kind of loss.

So that kind of gives me inspiration, because I know some of them speak up about this issue and some have chosen not to or just can't.

So that kind of keeps me going.

It can be very difficult.

Obviously, after 25 years and seeing progress made in some places, and, certainly, nationally not really seeing much progress until two years ago.

AMNA NAWAZ: Tell me about that progress.

I mean, as you know, the trend of gun violence in America has gotten worse in the last 25 years.

Where are we in our efforts to end gun violence here?

How would you assess this moment?

TOM MAUSER: I think I would summarize it as we have seen a lot of progress made at the state level, and it's essentially in blue states, and we have seen nothing done, in fact, we have actually gone backwards at the national level since Columbine, up until two years ago, when they passed the bipartisan Safer Communities Act.

And it took almost 30 years to get something significant like that.

I feel grateful that we have that.

I think it's great that we have seen finally some legislation passed.

But I also feel that, well, it took 25 years after Columbine.

We shouldn't have to wait another 25 years for something significant to pass out of Congress.

AMNA NAWAZ: And how do you view that tension between what we see at the state level and the lack of action at the federal level?

What do you want to see happen in terms of specific steps?

TOM MAUSER: I think, certainly, the states that don't have, for example, a red flag law really need to have one.

I mean, it's especially one step that you can take for when you -- when someone is identified as someone who is potentially a harm to themselves or others.

It's the simplest thing we can do.

And yet it's so frustrating to see that a number of states haven't done that, and we don't have it at the national level.

AMNA NAWAZ: And you see, of course, different responses from different states.

Tennessee, for example, in response to a mass shooting they suffered last year, the legislature just passed a bill this week that allows teachers and staffers to carry a concealed weapon in school after they get some training.

There's been some criticism.

There's been some people who say this could actually save lives.

How do you view something like this?

TOM MAUSER: I think it's a step backwards.

I really am opposed to arming teachers, for a few very simple reasons.

One, accidents can happen.

Two, when you have somebody at the scene who's holding a firearm, if you have a number of them holding a firearm, the police don't know who the good guys and the bad guys are.

Also, there can be crossfire.

You can make a situation worse by introducing a firearm if, for example, the potential shooter hasn't started shooting yet.

There's just many kinds of things that can go wrong.

AMNA NAWAZ: As you well know, if Daniel had lived, he would be 40 this year.

And I imagine that this work allows you to honor his memory as much as you do, but it also must keep you closer to the pain of losing him.

I wonder if you ever think about stopping and stepping back.

TOM MAUSER: No, not really.

But you're right.

It is painful.

People -- I think many people think that, when you do something like this and you're honoring your son, dealing with this issue, that it's somehow healing.

But, yes, I mean, every time I talk to a group of people, I'm talking about what happened to me and what happened to Daniel; 25 years later, I still get choked up when I do that most of the time when I'm speaking, because it is a reminder.

It is a trigger for what happened.

AMNA NAWAZ: What do you want us to know about Daniel?

TOM MAUSER: I want that -- people to know that Daniel was a -- just a wonderful, sweet kid, Boy Scout, played piano.

He was very shy and very inquisitive.

And I think what I most admire about Daniel is, he took on his weaknesses.

He was not at all athletic, and yet he chose to join the cross-country team at Columbine.

He was so shy, and yet he chose to join the debate team at Columbine, where he had to get up in front of other people and speak.

And, really, that's been the inspiration for me, because I feel like I'm also introverted, and yet, if he can do it, I can do it.

AMNA NAWAZ: Tom Mauser, thank you for the gift of your time and for sharing more about Daniel with us.

Thank you for joining us.

TOM MAUSER: Sure.

You're welcome.

Thank you.