Twenty years ago, two armed gunmen entered Columbine High School and, in just under an hour, killed 12 students and one teacher.

While the shooting was not the first in an American school, the trauma of that day sent shock waves through the US. For the first time, news crews were stationed outside of a mass shooting at a high school for weeks, providing 24/7 coverage. Calls for changes to America’s gun laws were made.

Since those 13 deaths on 20 April 1999, here is what we have seen as America’s gun violence epidemic has continued on almost uninterrupted.

In the days, weeks and years since Columbine, 226,000 children have been exposed to gun violence while at school, according to an analysis published by The Washington Post, illustrating the vast reach of a shooting epidemic that scars those who are fortunate enough to escape with their lives.

That exposure has occurred in least 233 schools where incidents of gun violence have taken place in the past 20 years, according to that Post analysis.

Sandy Hook elementary school shooting 2012 Show all 5 1 /5 Sandy Hook elementary school shooting 2012 Sandy Hook elementary school shooting 2012 con0.ap.jpg Scenes of shock and devastation at the Sandy Hook Elementary School in Newtown, Connecticut AP Sandy Hook elementary school shooting 2012 v2Connecticut.jpg Picture from local newspaper the Newtown Bee shows Connecticut State Police lead children from the Sandy Hook Elementary School AP Sandy Hook elementary school shooting 2012 con1.reut.jpg The scene outside the primary school following the incident REUTERS Sandy Hook elementary school shooting 2012 con2.reut.jpg Concerned parents picking up their children following the shooting REUTERS Sandy Hook elementary school shooting 2012 obama.reut.jpg Obama wipes away a tear during the press conference REUTERS

During those shootings at schools, some 143 children, educators, or other individuals were killed in the gunfire.

Those shootings had a much higher injury rate, however, with 294 injuries across those 20 years.

While press coverage of mass shootings at schools and colleges in America often focuses on instances where there are white victims, black students are overwhelmingly impacted. While black students make up 16.6 per cent of the student population in American schools, they represent 33 per cent of the victims. White students, meanwhile represent 56.7 per cent of students in US schools, but 38.1 per cent of the victims.

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Since Columbine, there have been three school shootings with a higher death toll than the 13 in Littleton, Colorado. The shooting at Marjory Stoneman Douglas High School in Parkland, Florida, last year left 17 students and faculty dead.

The shooting at Sandy Hook Elementary School in Newtown, Connecticut, in 2012 left 28 dead – including 20 children. The shooting at Virginia Tech in 2007 left 33 dead.

While school shootings remain relatively rare in the United States, there was an up-tick in 2018. That year there were at least 25 shootings at schools across the US – including the deadly attack in Parkland.

The epidemic facing America has fuelled a $2.7bn market for school hardening infrastructure, with that approach being a go-to response for politicians who do not want to cross the National Rifle Association.

That figure doesn’t even take into account the money spent on police officers hired to patrol schools under the auspice of keeping students safe – and some studies have indicated that school hardening might not be as effective as one would hope.

More than 1.3 million would-be firearm owners have been denied access to guns since 1998, when the FBI launched its background check programme. That’s out of more than 287m background checks conducted by the organisation.