Look Closely for Evidence of First Mass Shooting Happened 20 Years Ago at US Columbine High School

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."

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."