- Report by ITV News producer Lewis Denison
On the late morning of April 20, 1999 pupils Dylan Klebold and Eric Harris walked into Columbine High School, heavily armed with guns and bombs.
While gun violence had seen dozens of students killed in the century before the massacre, their horrific murder of 13 people was, at the time, thought to be the deadliest school shooting in US history.
It has since been surpassed by more deadly acts of school violence.
Yet 20 years on, the mere word Columbine still has the power to shock.
Its impact is perhaps magnified by the lasting memories of the atrocity, because almost all of the teenagers' attack was caught on camera.
- What exactly happened at Columbine?
Klebold, 17, and Harris, 18, drove in separate cars to their school in Littleton, Colorado with the intention of killing potentially dozens of their fellow students.
The pair walked into the school's cafeteria and place two duffel bags each containing a 20-pound propane bomb, which they had set to explode at 11:17 am.
The teenagers then headed back outside to their cars and waited for the bombs to go off.
When no explosion came, they began their murderous rampage, shooting students outside the school.
Within less than 20 minutes, Klebold and Harris had killed 12 fellow students and a teacher and wounded 21 others.
They then turned the guns on themselves and were found dead by police in the school library, along with 10 of their victims.
In chilling CCTV footage the pair could be seen laughing and, as Guardian journalist Andrew Gumbel said 10 years on, looking "as though they were having the time of their lives".
Possibly more shocking than video of the massacre is the level of meticulous planning which went into it - most of which was also captured on video and documented in journals.
- Leading up to the massacre
The Columbine massacre was first intended to be much more than a school shooting.
Evidence from Harris's website and both boys' journals show they hoped to blow up the entire institution.
That plan - to blow up hundreds of students in the school cafeteria and the library above, before using their guns to kill fleeing survivors - was at least a year and a half in the making.
The two had been friends for some time before the massacre but it appears their involvement with each other took a more sinister turn when they were arrested for stealing items from a van in January 1998.
They both pleaded guilty and were sent to a juvenile diversion programme.
Then, two months later in March, the parents of a student at their school filed a report with the sheriff's office, stating Harris had threatened to kill their daughter and had written on his website that he would like to kill people.
Despite this, and the fact Klebold wrote a school essay two months before the attack about a man carrying out a massacre and enjoying it, authorities failed to act.
Then, in the weeks before the massacre, Klebold and Harris began recording home videos in which they bragged about their plan to "kill 250" - in one they even apologise to their parents for what they plan to do.
Six weeks before the killings the pair filmed themselves in a forested area shooting at bowling pins.
It has been reported they hoped to emulate destruction caused by the 1995 Oklahoma City Bombing and it is thought they originally planned their massacre to take place on its anniversary: April 19.
Instead, reportedly because they could not access ammunition in time, the plan was carried out a day later.
While the boys were condemned around the world, anger mounted in other directions too.
- Who else was blamed?
At the time authorities and the media blamed the massacre on a wide range of factors including the pair being bullied, their love of violent video games and, most memorably, musician Marilyn Manson.
Most of these theories have been discounted.
It was reported the troubled teenagers were obsessed with "Doom", a first-person shooter video game in which the aim is to kill enemies.
In a video recorded before the shooting Harris says the massacre is "going to be like f*****g Doom".
Another theory is the pair were part of a group of goths at school called the Trenchcoat Mafia who allegedly worshipped Marilyn Manson and were bullied by other kids at school.
The pair did indeed wear trenchcoats during the spree but it was incorrectly reported at the time they were also wearing Manson clothing.
The violent themes in Manson's lyrics led much of the media to say he inspired the killers however it later transpired the pair were not fans of his music.
It also later turned out the pair were not members of the Trenchcoat Mafia and were not as bullied as early media reports had speculated.
Inevitably, others turned their ire on the gun lobby.
- What was the National Rifle Association's reaction?
The power of National Rifle Association (NRA) remains a fundamental reason why 20 years on guns are still widely legal in America.
Interestingly, what its then president, Hollywood star Charlton Heston, said about Columbine at the time is strikingly similar to what the NRA said following more recent school shootings.
In 1999, Heston said: "If there had been even one armed guard in the school he could have saved a lot of lives and perhaps ended the whole thing instantly.".
In 2018, following the Parkland school shooting, NRA vice president Wayne LaPierre said it was "completely ridiculous" to increase gun control to avoid further deaths.
Instead, echoing Heston, he suggested: "Put armed security in every school, fix the broken mental health system."
LaPierre had also responded to gun criticism in 2012, saying: "The only thing that stops a bad guy with a gun is a good guy with a gun."
- How has school violence affected America since Columbine?
In the aftermath of Columbine a huge gun control debate was sparked across the US and many schools enacted a "zero tolerance" approach to threats of violence.
Despite the shooting and subsequent gun control debate, the right to bare arms continues to be enshrined in the US Constitution and hundreds have died in similar incidents since.
It means Columbine is no longer the deadliest school shooting. Two decades on, it's the fifth.
The deadliest school shooting remains Virginia Tech in 2007, in which Sueng-Hui Cho killed 33 people.
But gun violence remains a tragically common threat across the nation.
In 2018 a study by the Washington Post found 187,000 students in at least 193 primary or secondary schools had experienced a shooting on campus during school hours since Columbine, according to the year-long analysis.
That number only rises with each year.