Twenty years later, Dawn Anna still saves a seat for her daughter, Lauren Townsend, when she goes to the movies, knowing full well that she won’t show.

Twenty years later, Frank DeAngelis' morning mantra consists of reciting the names of 12 students and a teacher he will never see again.

Twenty years later, Tom Mauser literally walks in the size 10½ shoes of his slain son, Daniel, whenever he’s called on to speak about the unspeakable.

For them and for too many others, the massacre at Columbine High School is both past and ever-present, a wound that never really heals because every time somebody with a high-powered weapon unleashes a bloodbath at a school, the scab gets ripped off.

"It seems like every month, there’s a new tragedy of some kind somewhere around,” said Rick Townsend, whose daughter, Lauren, was 18 when she was gunned down. “It just makes you feel sometimes hopeless.”

Columbine has been the target of threats over the last two decades, the most recent incident involving Sol Pais, 18, who authorities said had exhibited an "infatuation" with the shooting. The Florida teenager traveled this week to Colorado, where she bought a gun and fatally shot herself Wednesday as authorities were searching for her.

Experts say copycat shooters often become obsessed with the date of a tragedy that they hope to emulate.

With the 20th anniversary of the mass shooting looming, Townsend and several other grieving parents and survivors sat down with NBC Nightly News to talk about what has changed and what, sadly, has not.

“When there is another school tragedy, which just rips at our hearts every time we see that, the word Columbine will be brought up,” Anna said. "This is an amazingly strong, loving community, and Columbine, that word Columbine, should mean that. United."

“We know exactly the feeling that these families are gonna feel for the rest of their lives, that hole that’s gonna be in their hearts,” Bruce Beck, Lauren Townsend’s stepfather, said.

Coni Sanders, daughter of Dave Sanders, the slain teacher, said what happened at the Colorado high school changed everything and yet nothing.

“I feel like we have come so far in so many ways, yet we’re still stuck in the same spot,” she said. “I never imagined that we would be where we’re at right now, where there are so many mass shootings that we can’t even keep up … it’s just unfathomable that Columbine wasn’t enough.”

The Washington Post, using law enforcement reports, news articles and various databases, has calculated that as of April 8, more than 226,000 students “have experienced gun violence at school” since Columbine.

Of those, 143 children, teachers and others were killed and another 294 were injured, according to the newspaper’s tally.

The tragedy at Columbine began unfolding at 11:19 a.m. on April 20, 1999, when two troubled students, Dylan Klebold, 17, and Eric Harris, 18, opened fire outside the school, then marched inside, making for the library.

By 11:35 a.m., they had taken 13 lives and wounded more than 20. All their victims were chosen at random, it was later revealed. By noon, both shooters were dead after turning their guns on themselves.

A previously obscure Denver suburb called Littleton was now the epicenter of a national tragedy.

Images of heavily armed SWAT teams descending on a school and the sight of students filing out with their hands up were burned into the national consciousness.

“Columbine played out on TV,” Beck said. “No previous school shooting had done that. There was the unknown of where the shooters were during the entire time that it was being filmed, so I think people connected with Columbine more.”

In the aftermath, many schools beefed up security, began holding lockdown drills, and introduced “zero tolerance” rules meant to thwart massacres by cracking down hard on students who threaten violence.

New programs were developed to prevent bullying and help social outcasts after it emerged that Harris and Klebold, both gifted students, had been picked on for years.

More than a decade later, Klebold's parents confirmed in a book that their son was an outcast and revealed that police told them during the shooting that he was a suspect.

"And so while every other mother in Littleton was praying that her child was safe, I had to pray that mine would die before he hurt anyone else," Sue Klebold said in the book "Far From the Tree."

Sean Graves, one of the survivors, has said in previous interviews that he knew the shooters but was not close to either of them. He said he was across the street from the school with his buddies when the bullets began to fly.

Shot six times and left partially paralyzed, Graves became an inspiration to many when he climbed out of his wheelchair, leaned on a crutch and walked across the stage to collect his high school diploma in 2002.

In anniversaries that followed, Graves would visit the spot where he was shot and light a cigar in memory of his friend, Danny Rohrbough, 15, who was killed there.

“It’s hard to imagine it’s already been 20 years,” a visibly emotional Graves told NBC News. “I was 15 years old when I was shot. ... It’s hard for me to picture life, what it was before.”

Graves said everybody who was at Columbine that day was a victim, not just the people who were hit by the gunfire.

“You’ve got people that were physically injured, you got people that lost their lives,” he said. “And you’ve got people that were emotionally scarred for the rest of their lives. And I want people to know that it’s OK to need to heal. Some people never heal. I’ll physically never heal, but emotionally I think that I’m on the right path.”

Graves said he's both anxious and excited by the anniversary.

"I think this year, it's gonna be different because we're spreading the word of both hope and positivity, and love and commitment to give back," he said. "To me, that would mean a lot more for this anniversary to move forward in a positive vibe than anything else."

Rachel Scott, 17, was the first person shot in the massacre. She was eating lunch with her friend, Richard Castaldo, on the lawn outside the school when she was killed. Castaldo, who was also hit and played dead while the killers moved on, was left paralyzed.

In the aftermath, Rachel’s father, Darrell Scott, founded Rachel’s Challenge, a campaign aimed at ending bullying in schools. He also was one of several activists invited to the White House to meet with President Donald Trump after the Feb. 14, 2018, massacre that claimed 17 lives at Marjory Stoneman Douglas High School in Parkland, Florida.

Darrell Scott said he frequently gets asked what he would advise other parents who have lost children in mass shootings.

“It’s very simple,” he said. “First of all, I’m sorry for your loss. And second, it would be, celebrate the life of your child. And if you can, find it in your heart, practice forgiveness, because unforgiveness creates bitterness, anger, a need for revenge and can ruin your life.”

Does he forgive the killers?

“Yes,” Darrell Scott insisted. “There’s a difference between forgiveness and pardon. I wouldn’t have pardoned Eric and Dylan. I would have prosecuted to see they could never do it again. Forgiveness is not for them. It’s for me. “

Craig Scott, Rachel’s brother, was in the high school library when the shooters barged inside. He saw his friends Isaiah Shoels and Matthew Kechter get fatally shot. He found out the next morning his sister also was dead.

Columbine, he said, “was where I lost Rachel and where I saw my friends die.”

“It’s a place where my life changed forever,” he said.

Craig Scott, who was a sophomore at the time, first spoke publicly about what happened in a wrenching "Today" show interview during which he and Shoels’ father held hands to comfort each other. He said in the years that followed, he resisted being defined by that tragedy.

“For a long time, my whole identity got wrapped up into that,” he said. “And I had to break away.”

Now, however, Craig Scott said he has found his calling — talking about what happened as part of a Denver-based program called Value-Up.

“I’m thankful because it has made me who I am now,” he said. “And it has given me a platform to be able to speak now into kids’ lives and make a difference in the here and now.”

Tom Mauser also found his voice after his 15-year-old son, Daniel — a straight-A student and a member of the school debate team — was killed. He became a dedicated gun control advocate and wrote a book about dealing with his grief and his campaign against the National Rifle Association. It's titled “Walking in Daniel’s Shoes” because, well, he does.

“It was a number of weeks after the tragedy that we were cleaning out some of his personal effects and I came across a pair of tennis shoes,” he said. “And I asked my wife, ‘What size shoes does, did, Daniel wear?’”

Told they were size 10½, Mauser recalled saying, “That’s my size.”

“It suddenly occurred to me, wow, I could be wearing his shoes and walking in his shoes,” he said. “I’m taking his place on the debate team at Columbine, arguing this great issue about gun violence.”

Mauser wears Daniel's sneakers whenever he speaks publicly about the tragedy that still breaks his heart.

“I feel that it gives me strength,” he said.

Mauser said Australia and New Zealand went after guns when those countries experienced mass shootings, and he has no patience for politicians who are quick to offer up platitudes when it happens in the United States but won’t push for gun control.

“It’s the proverbial thoughts and prayers and really nothing else,” he said. “I think to really honor people and to say we’re really gonna resolve to do something about it, it has to be more than thoughts and prayers.”

DeAngelis, the Columbine High School principal that day, waited until 2014 to retire, saying he felt a moral obligation to stay until those who were in elementary school at the time of the shooting graduated.

He said he often thinks about the students who were killed, about what kind of grown-ups they would have been.

“They’d be young adults, 38 and 39,” he said. “And their lives were taken far too quickly.”

So he’s found his own way to honor them — and comfort himself.

“Every morning I wake up, I recite the names of the 13, because they’ll always have a special place in my heart,” he said. “And I’m gonna keep their names and their lives alive as long as I can.”