On this day in 1999, 12 students and one teacher were murdered at Columbine High School in what was at that time the United States’ worst school shooting.
The killers were two senior students Eric Harris and Dylan Klebold, who then took their own lives in the school library.
The pair had also planted home-made bombs around the school campus, including in the cafeteria and their cars in the school car park, but the bombs failed to detonate.
They made videos before the attack talking about what they were going to do and apologising to their parents for it.
At the time, it was the deadliest school shooting in US history, devastating the school community and the entire nation.
The crime triggered heated public debates about gun control laws, American gun culture, bullying, social outcasts and violence in video games.
Many of these arguments are still raging today.
In the case of Columbine High School, police were criticised for being too slow to enter the school and not intervening.
SWAT teams entered the school 47 minutes after the shootings started, and five hours passed before law enforcement declared the school under control.
The backlash to police response resulted in the introduction of the Immediate Action Rapid Deployment police tactic, where first responding police officers are given the dangerous task of instantly finding the shooter rather than waiting before entering a school.
No one knows why Harris and Klebold embarked on their shooting rampage, but authorities believe the pair had planned the massacre for about a year.
Their sickening crime has inspired several copycats among others who view Harris and Klebold as martyrs.
This has become known as the ‘Columbine effect’.
Mass shootings are now so common in the US that in 2019, mass shootings outnumbered the days in the year.
By the end of 2019, there were 417 mass shootings in the US, according to data from the non-profit Gun Violence Archive (GVA).
Columbine High School is still in operation today, welcoming hundreds of students.