Evan Todd still has sharp memories of April 20, 1999. As a sophomore at Columbine High School, he was in the library with his friends on that cool, sunny morning, attempting to write a paper but really just goofing off, throwing around wads of paper.

In an instant everything changed. Todd described an explosion, smoke, and then pops of gunfire echoing through the hallways. He felt a rush of adrenaline as panic set in around him.

"I remember thinking, someone with a gun has got to come in here like any day, you know," Todd said. "You just think they're going to come in and stop these guys and we're gonna get out of here. And, nothing."

Todd watched as the two killers murdered his classmates, execution-style.

"One of them kneeled down and put a gun to my head and said, 'Why shouldn't we kill you?'" remembered Todd. "And so when they came up to me, I really thought, this is it, this is the end of my life."

Credit Leigh Paterson / KUNC / KUNC Plaques with messages dedicated to the victims of the attack circle a memorial near Columbine High School.

The killers didn't pull the trigger. Todd survived.

His experiences that day have informed his thinking on safety and guns. Todd now believes a teacher who is allowed to carry a firearm can make a difference during a school shooting. He is a gun owner and a concealed carry permit holder himself. Earlier this year, Todd testified at the Colorado statehouse in favor of a bill that would allow people to carry guns on school grounds and has done some public speaking on the issue .

"What actually stops these from happening? And in the world we live in, a firearm is one of those ways," said Todd, speaking about school shootings. "And a firearm would have saved lives at Columbine."

Credit Leigh Paterson / KUNC / KUNC Columbine survivor Evan Todd makes lunch as his young son pulls on his shirt. Todd has become even more passionate about school safety since becoming a father.

Arming teachers

Research on the effects of armed teachers in schools is sparse, but according to an FBI analysis of 160 active shooter situations in many types of locations, an armed civilian or security guard stopped just five of these incidents.

Experts who are critical of teachers carrying guns say that this trend could result in dangerous situations such as accidental firings and gun thefts all in the name of solving a problem that is rare: mass shootings account for less than 2 percent of gun deaths in the U.S .

Regardless of data, though, the fear that Todd experienced at Columbine is driving policy in many parts of the country. The national outcry and media attention after mass shooting incidents has galvanized responses across all political lines.

Currently, teachers or other school staff in districts in 31 states can legally carry weapons in schools, according to a review of state laws and local news coverage by Guns & America.

Luis Melgar & Matt Richmond/Guns & America

Some states have set policies, but often because of pressure from parents to do something, some local school districts have made their own decisions. There aren't any federal standards but recently, there has been conversation about whether or not federal money can be used to buy guns for schools.

"The truth is we are never going to be 100 percent perfect and have risk elimination in schools or anywhere else in our society," said Ken Trump, president of National School Safety Services and an expert on emergency preparedness. He thinks arming teachers is a mistake.

"What is interesting to me is that we've spent as a nation the past five or 10 years increasingly being critical of law enforcement," he said. "And we've demanded more training, more scrutiny. And at the same time now, we were having conversation where we say give somebody much less training compared to a police officer. Put him in a school and, and everything's going to be better."

Other mass shooting survivors, even survivors of Columbine , have had the exact opposite reaction from Todd.

For Todd, though, the potential benefits simply outweigh the potential risks.

"Had I not gone through it, I don't know if I would have the same perspective," said Todd. "And that's that difficult part because I can see that side of it if you hadn't gone through something. People carry on because they know how rare and how safe their children are. But I've seen evil in this world. And ignoring it never does anything."

In 1999, the nation was shocked by a school shooting at Columbine High School. Columbine Then & Now explores what's changed and what has stayed the same over the past two decades.

Lisa Dunn contributed to this reporting.

Guns & America is a public media reporting project on the role of guns in American life.