On April 20, 1999, two students opened fire at Columbine High School in Littleton, Colorado, gunning down 12 of their fellow students and a teacher before killing themselves.
America endured school shootings before Columbine, but never "one quite like that one," said ABC News contributor and former FBI agent Brad Garrett.
The sheer shock of 13 people losing their lives in chaos that unfolded on live TV launched the country into a new era -- and new century -- of school shooting coverage, ahead of Virginia Tech, Sandy Hook, Parkland and countless others.
'My worst nightmare became a reality'
Fifteen-year-old Columbine freshman Laura Farber was in the cafeteria when a janitor yelled for everyone to get under the tables.
She was sitting by the door, so she and her classmates rushed out, running through the parking lot to a residential street, where they knocked on strangers' doors for help. Once Farber was safely inside a home, she heard shots across the parking lot.
Then-Columbine High School Principal Frank DeAngelis was in his office when his secretary ran in and said there was a report of gunfire.
"The first thing that crossed my mind was, 'This has to be a senior prank,'" he told ABC News.
But as he stepped into the hall, "My worst nightmare became a reality." About 100 yards away was a gunman coming toward him.
"Everything seemed to slow down," he recalled. "I remember distinctly shots being fired, glass breaking behind me. And I remember thinking, 'What was it gonna feel like to have a bullet pierce my body?'"
That's when a group of girls came out of a locker room, walking unsuspectingly down the hallway. DeAngelis herded them into a gym storage area. He peeked outside and saw a sheriff's officer, so went back for the girls and rushed them out of the building.
'That strategy of waiting seems nuts'
DeAngelis tried to get back inside the school, but officers said he couldn't -- they had to secure the perimeter and wait for SWAT.
"Today, that strategy of waiting seems nuts. But it was the protocol of the time," DeAngelis wrote in his newly-released book.
Another question of protocol concerned the school resource officer, with whom the gunmen exchanged gunfire outside before storming the school, DeAngelis said.
Following protocol, the officer didn't enter the building. But if the officer had followed them inside, "there's a good chance" the gunmen wouldn't have reached the library, where so many classmates were targeted, DeAngelis said.
"One of the big issues in Columbine was law enforcement wasn't trained to go after the shooter," said Garrett, the former FBI agent. Instead of immediately confronting the threat and racing into the building, police secured the scene and waited for SWAT teams to arrive, which allowed the gunmen to continue to fire inside.
Forty-eight minutes ticked by at Columbine before SWAT entered the building, DeAngelis said, as the officers first had to get their gear at their precincts -- leaving DeAngelis and the police at the scene feeling helpless.
In the two decades since Columbine, first responder protocol has drastically improved.
Now, most police departments have rapid response officers who carry heavier assault weapons and are trained to enter immediately and "follow the firepower," said Garrett. "If they hear shots, they're going to keep moving closer and closer to it until they confront the shooter."
Security and preparedness
While gunshots rang out, Columbine teachers were forced to open classroom doors and reach around to lock them, "putting themselves in harm's way," DeAngelis said, as the doors only locked from the outside.
Now, schools are built to have classroom doors lock from the inside, he said.
U.S. schools have undergone many other security and preparation upgrades these last two decades, and in the wake of Columbine, local police started storing school blueprints to help map out response plans.
Though schools have multiple doors, now they're commonly set up so students only use certain ones to get into the building, setting up a "choke point" that "security people can control as they come into school," Garrett said.
Over 90% of schools now have a written crisis plan, Garrett said, while over 75% of schools -- as young as elementary -- hold active shooter drills.
Drills weren't typical before Columbine, and now, active shooter scenarios are a common conversation for kids to have in school, whether they live in a rural setting or urban setting, Garrett said.
"The simplistic training is 'run, hide, fight,'" Garrett said. Some schools teach students about the best hiding places in the classroom, how to barricade the door and how to throw objects at an intruder as a distraction, he said.
Though new measures are in place in schools, mass shootings overall aren't on the decline, Garrett said.
However, there are upsides: due to the new protocols, school shootings are usually shorter, lasting just a few minutes until the gunman is confronted. And thanks to new security measures -- like surveillance cameras -- overall crime in schools, including theft and sexual assault, has decreased, said Garrett.
Mental health: Struggles and stigma
When Farber, the Columbine freshman, got home the night of the massacre, she hadn't yet processed or learned the full facts of what happened.
"It really felt like we were going to go to school the next day," and she figured she needed to do her homework, Farber recalled. "You're so immature and innocent."
Farber wanted life to go on "as normal as possible," but "there's just always this underlying feeling of guilt," she said. "Could I have done something to make this not happen?"
DeAngelis, too, wrote in his book, "I had a difficult time looking at people in the eyes because of the guilt."
The principal spent his post-Columbine years wading through uncharted waters as he tried to help himself, his staff and his students through the trauma.
"Many of the kids at Columbine said, 'I'm fine, I'm fine, I don't need to talk to anyone,'" DeAngelis said. "The most difficult thing was just providing support and help to everyone."
Each student dealt with the tragedy in a different way, he said. DeAngelis soon realized Columbine could no longer serve Chinese food because that was served the day of the shooting. Some students wouldn't participate in fire drills. The school couldn't show war movies, he said, and the administration banned camouflage clothing because first responders wore it that day.
"As we got back into the groove of school, I repeatedly emphasized the need to be open to getting help," DeAngelis wrote in his book. "I'd say to teachers or kids, 'I don't know about you, but gosh, I'm having a hard time sleeping, I have no appetite, and I'm having these bad dreams.'"
The first few years after the shooting, DeAngelis said his weight dropped and he went to the emergency room several times, mistaking anxiety for a heart attack.
Despite the overwhelming trauma, DeAngelis noticed an immense stigma surrounding mental health treatment, and he said he was discouraged from disclosing he was seeing a therapist in case he'd be "deemed unfit for duty."
"I got the help I needed," he stressed. "And that was so important."
DeAngelis, who spent 18 years as principal, retired in 2014 -- after he felt he had done his duty to heal the community.
One thing that hasn't changed is that school shootings are still happening, with one after another unfolding in the 20 years since Columbine.
At least 143 people have been killed in school shootings since Columbine, according to The Washington Post (school shootings are not tallied by the federal government, The Post notes).
Since Columbine, there have been 11 school shootings that were considered mass shootings -- where four or more victims were killed. Three of these shootings -- Sandy Hook, Parkland and Virginia Tech -- were deadlier than Columbine. The massacre at Virginia Tech in 2007 is the deadliest school shooting in U.S. history.
Some captured the nation's attention: the horror of the mass murder of 20 young children and six educators in the 2012 Sandy Hook Elementary School shooting in Newtown, Connecticut; the 2018 shooting at Marjory Stoneman Douglas High School in Parkland, Florida, where 17 were gunned down, allegedly by a former classmate, sparking a student-led revolt for gun reform across the country.
But other school shootings barely made a blip on the national stage.
Weeks before the Parkland massacre, two teenagers were killed and over a dozen people were hurt in a shooting at Marshall County High School in Benton, Kentucky. One month after Parkland, a boy shot two classmates at Great Mills High School in Maryland. A 16-year-old girl -- the gunman's target -- was killed. The second victim survived.
Sometimes after a high-profile mass shooting, states will tighten up gun laws, requiring background checks, restricting the age for buying guns, reducing the sale of assault-type weapons or banning the sale of high capacity magazines, said Garrett.
But in other cases, states will loosen laws after mass shootings to make it easier to buy guns and get permits.
States have had the most control over gun laws, Garrett said, with very little movement at the federal level. (Garrett considers one of the most significant federal changes in recent decades to be the assault weapons ban signed by President Bill Clinton in 1994, which expired in 2004 and was not renewed by Congress.)
But these state-level restrictions haven't had a big impact on mass shootings, Garrett said, "because mass shootings are not impulsive. These [gunmen] will plan weeks, months, sometimes years, in advance as they build up. So if you restrict the way they buy weapons, they can just wait or go to another state to buy an assault weapon."
Meanwhile, just as a new school shooting reignites the gun control debate, it also introduces a new, shocked community to Columbine's unfortunate club, and as the leaders of the pack, Columbine survivors feel a duty to pay it forward.
Farber is now a filmmaker and released a documentary this month in which she followed several of her fellow survivors as they returned to the Columbine room where they were when gunfire erupted.
They reflect on how surviving transformed them from carefree teenagers to terrified and bitter young adults. But in the last 20 years, they've also found meaningful careers, life partners and ways to cope.
Farber hopes the film will reach trauma survivors beyond Columbine and help them move past their own experiences.
Other Columbine survivors founded The Rebels Project, a non-profit that connects mass shootings survivors with each other to help find a support system.
Parkland students visited Colorado last year to meet Columbine survivors, DeAngelis said, and the elders gave the teenagers guidance on how to maneuver their years ahead.
"There's that instant credibility, which I think really helps," DeAngelis said. "I'd be willing to bet that these Parkland kids will be doing the same thing if these tragedies happen down the road."
But when it comes to preventing these future tragedies, DeAngelis hopes people know "there are kids that need help in schools."
"And it worries me when I hear communities and government agencies talking about cutting out counselors in the building, cutting out psychiatric help, cutting out mental health support," he said. "Then you add the social media aspect -- making sure that we see what is going on with the kids' social media."
The key, he said, is those pieces working together as one: Prior to Columbine, he said, law enforcement, mental health professionals and school administrators worked separately.
"When you take those different pieces of the puzzle, put them together," he said, "I think we have a better chance of stopping these things."
ABC News' Meghan Keneally contributed to this report.