Columbine survivors explain what really happened during the infamous attack

School shootings have become a sad fact of life in the US, but perhaps none are quite so infamous as the Columbine massacre.

The incident, which unfolded in 1999, became somewhat of a blueprint for future school shooters and is often referenced in the media.

On April 20 that year, students Eric Harris and Dylan Klebold fatally shot 12 students and one teacher at the Colorado high school before turning the guns on themselves.

They had originally planned for the attack to be primarily a bombing, but their homemade explosives failed to detonate.

Though there have been countless reports and films depicting the tragic events, a number of the real survivors have explained what it was really like on that fateful day.

In the crime docu-series Killing Spree, Sean Graves, who was just 15 years old at the time, recalls the moment he and his friends Dan and Lance were shot at by the two killers outside of the school cafeteria.

He says: "My body was completely behind a concrete wall so I was protected. The only thing on me that was sticking past that was my backpack.

"A bullet entered the side of my backpack, spun around and actually shot me in the back and shot out my right hip.

"At that moment, I lost my legs completely. I was paralysed from the waist down immediately."

Sean grows emotional as he remembers the shocking attack, with the narrator explaining that he initially thought it was a prank.

He continues: "I found myself laying halfway in the school and halfway out, and at that moment I knew that my worst nightmare had come true.

"Months, possibly years before the shooting actually took place I had a recurring nightmare of somebody in my family or even myself being paralysed.

"It didn't make any sense then, but the second I looked back at my legs, I realised that dream had come true."

Killing Spree/True Crime Network

At that moment, Sean explains that all he can remember is the 'fear' in the other students' eyes as they realised what was happening.

CCTV footage from the cafeteria shows the moment the fire alarm goes off and everyone begins to panic, clambering over each other to escape.

Former Columbine High School principal Frank DeAngelis also shared his experience of that day, saying the secretary came running in to tell him there had been reports of guns firing.

"I came out of my office and I ran down here and, as I looked down the corridor, I saw a gunman coming towards me with a shotgun," he explains.

"When I was staring down and I made contact, it seemed like the size of a can. All I kept thinking is what it was going to feel like to be shot? I thought of my family, was I going to die quickly?

"Each time a bullet was fired from the gun, glass would shatter behind me."

DeAngelis went on to say that what probably saved him was the moment a group of students came out of the gymnasium.

Killing Spree/True Crime Network

Seeing that they were walking towards the shooter's crossfire, the principal ran over and ushered them back into the gym.

The shooters started their attack at around 11.15am, and by 11.27am they were in an area by the library where former student Craig Scott and a number of his peers were working.

Just like Sean, Craig initially thought it was a prank when they heard what they thought were 'firecrackers' going off.

"We're laughing, we're joking about it, and then that's when a teacher ran into the room, she ran over to the phone, she got on the phone to call the police," he says.

After playing the frantic 911 call, Craig explains that he got under a table alongside his friend Matt and Isaiah, adding: "As soon as they came in, they were shooting off their guns.

"They began to yell at students, they were making some mocking comments... the first person that they killed was Kyle Velasquez who didn't know to hide. He had a disability."

Craig continued to hide with his friends, saying a lot of students were being quiet or crying, while some were begging for their lives.

"They treated it like it was a game, like they were having fun," he says. "At one point they would pop underneath a table, they would say something like 'peekaboo' and they would shoot someone."

Killing Spree/True Crime Network

When the killers reached Craig's table, they targeted Isaiah Shoels, with Craig saying they called him a racial slur before fatally shooting him and his friend Matt Kechter.

The reason they didn't kill him, he believes, is because he chose to pretend to already be dead by laying face down on the ground.

Despite the ripple effect the shocking attack had, there have been countless copycat mass shootings since then.

Each time, it casts a grim shadow over America's gun law policy and the huge issue the country still faces with gun violence.

If you have experienced a bereavement and would like to speak with someone in confidence contact Cruse Bereavement Care via their national helpline on 0808 808 1677