Today at Columbine High School in Littleton, Colorado, you have to look closely for evidence that one of the worse school shootings in U.S. history happened here 20 years ago.

The building looks like any of the thousands of American high schools across the country. The students hanging out in front of the school or practicing on the football field weren’t even born on April 20, 1999, when two student gunmen opened fire on their classmates, killing 12 students and a teacher.

But the markers are there — if you look closely enough.

Just over the hill from the school at an adjacent park is the Columbine memorial, which pays tribute to the people murdered in the massacre. The football stadium is named for Frank DeAngelis, the principal who led the school through the crisis and stayed on for another 15 years.

The softball field honors teacher Dave Sanders, who bled to death in a classroom while waiting hours for medical help.

Protocol at the time called for sheriff's deputies to wait outside and secure the perimeter until the specialized SWAT team could arrive. Columbine was one of the first mass shootings at a U.S. school, and it changed how schools and law enforcement prepare to confront an active shooter.

“In 1999, that’s what deputies did. They ran toward danger, and then they contained it. Now, the training is different,” says Jeff Shrader, sheriff of Jefferson County, Colorado, whose deputies responded to the incident at Columbine two decades ago. “They’re going to go to the shooter. They’re going to do everything they can to neutralize the threat. To identify it and to neutralize it so that hopefully more lives would be saved.”

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At Columbine, first responders were also hampered by poor communication and uncertainty about the school’s layout.

“One of the things we were able to do shortly after that was to put maps of schools in command vehicles so that they were readily accessible,” the sheriff says. “In our command operations center, we maintain those, but they were things that just weren’t thought of in that point in time.”

Responding quickly to a school shooter is not the only thing that's changed since Columbine.

Visitors used to be able to walk right into most American schools. Now, exterior doors are locked, and many schools use an intercom buzz-in security system. Interior classroom doors often lock now, allowing students and teachers to lock themselves inside. There is a film that can be put on windows to effectively render them bulletproof.

The U.S. Secret Service’s National Threat Assessment Center (NTAC) finds that school shooters often experience some sort of stressor — a setback, challenge or loss — leading up to an attack.

"They had difficulty coping with perceived injustice or bullying that was happening to them at the school, whether it was real or perceived,” says Dr. Lina Alathari, chief of the NTAC. “They had a sense that they were being bullied, and in a majority of incidents, these students were being bullied."

FBI investigators concluded the Columbine killers were not bullied.

Today, the Secret Service recommends that all schools establish a threat assessment team, made up of teachers, counselors and others, to identify students in distress or who might exhibit concerning behavior with the aim of stopping violence before it happens.

"There is no specific type of student who would carry out an attack,” Alathari says. “In a majority of cases, these were mainstream students. The most common performance, they were As and Bs. They came from different types of families, intact families, single family homes. They were popular. Some were loners. So, there really is no single profile that you can point to and say that is the type of student that would carry out an attack.”

Markers that threat assessment teams should look for include students whose grades decline, are experiencing suicidal thoughts or becoming more isolative, as well as other changes in behavior.

“This is when we need to be intervening as a community to offer that student assistance before it escalates to the point where they view violence as an option,” Alathari says.

To date, the NTAC has trained more than 100,000 school personnel, law enforcement and others with a stake in school safety on how to identify and assess and intervene with students of concern.

About 90% of U.S. schools have a plan for what to do in the event of a school shooting. Seventy percent of schools drill students using that plan, according to the U.S. Department of Education.

Columbine survivor Samantha Haviland was a 16-year-old junior when the shootings happened at her high school. Two decades later, she is director of counseling for Denver Public Schools, and she has concerns about how lock down drills impact students.

“To remind our students over and over again that you are not safe, you are not safe, you are not safe, is causing a lot of anxiety,” Haviland says. “And we have students who are jumping out of windows during drills because they think it’s real because someone dropped a book at the wrong time.”

She says there is a part of Post-Traumatic Stress Disorder called hyper-vigilance, where a person is always looking for exits, places to hide or how to escape, and keeping an eye out for who might try to hurt you.

“That hyper-vigilance takes up a lot of brain capacity and really decreases our students’ ability to focus on education when they’re in school. So, they’re not learning at the same rate that maybe you or I did when we went to school,” Haviland says.

In 81% of the school shooting incidents the NTAC studied, other students knew that the potential perpetrator was about to carry out an attack or was interested in doing so.

More schools are adopting procedures that teach students how to safely report suspicious behavior.

"It's important for kids to see something, say something, hear something, say something,” DeAngelis says, “but then we need to do something as adults, and we need to follow up, and that is a key component."