Two sisters who survived the Columbine High School massacre have told DailyMail.com how they were forced to step over the bodies of their dead classmates to escape.

Laura Hall, then a 14-year-old freshman, was having lunch in the cafeteria when two teenage gunmen attacked Columbine High School in Littleton, Colorado, on April 20, 1999.

Her 16-year-old sister Sarah Bush had been in math class when she heard an explosion and fled the school.

But she had to wait hours before knowing her younger sister had made it out alive. Laura ended up barricaded in a small choir office with dozens of other students for more than four hours before a SWAT team came to free them.

By then, Eric Harris and Dylan Klebold had killed 12 students and a teacher and had taken their own lives - making Columbine the deadliest school shooting in US history at the time.

Now, on the 20th anniversary of the killing, Laura, 34, and Sarah, 35, who both live in Eagle Mountain, Utah, have told DailyMail.com how the massacre changed their lives forever – but said they are proud of how far they've come, despite the trauma.

Sisters Laura Hall, 34, (right) and Sarah Bush, 35, (left) were inside Columbine High School in Littleton, Colorado, when two gunmen opened fire on April 20, 1999

Laura, then 14, (pictured at her graduation from Columbine High School) was having lunch in the cafeteria while sister Sarah, then 16, was in math class when the shots first rang out

Sarah (pictured at her high school graduation) was able to flee the school at the start of the gunfire, but Laura ended up barricaded in a choir office for four hours before the SWAT team freed them

Sarah said she was taking a math test, so her classroom was absolutely silent. 'Just a few minutes into it, we heard an explosion and it was big.

'It was a big, deep boom and it shook the floor a little bit,' she said. 'A couple of minutes later, the baseball coach for our school came running through the hall and he was opening doors and yelling.

'He opened our door and said, 'You guys need to get out now. Somebody's shooting.'

She made it to a soccer field opposite the school and watched as a SWAT team showed up. Later, she and a group of friends ended up in a stranger's home – watching the news and unable to get in touch with their families.

Sarah later went to a nearby elementary school where officials were checking off the names of those inside the school, where she was reunited with her grandfather.

'At one point I remember [victim] Isaiah Shoels' parents,' Sarah remembered. 'They came up to me and they said, ''Have you seen our son? Have you seen Isaiah?''

'I said, ''No, I'm sorry, I haven't seen him.'' I found out several days later that he had died.'

Eventually, Sarah's grandfather found her and took her to the public library where her mom was. 'We hugged and we realized that neither of us really knew where Laura was.

'We would watch lists of names, they would bring in a list of names of kids who had been accounted for and they'd put them on this whiteboard.

'We would just watch and watch and wait. Her name just wasn't showing up on any of these lists. It was terrifying.'

On April 20, 1999, almost 20 years ago to the day, Dylan Klebold, 17, and Eric Harris, 18, began a murderous rampage that left 12 students and one teacher dead

The cafeteria where Laura ran from is pictured after bombs and bullets rang out during the Columbine High School massacre 20 years ago

The 1999 shooting was the most deadly school massacre in US history until the shooting at Marjory Stoneman Douglas High School in Parkland, Florida in February 2018

Meanwhile Laura had been eating an early lunch at around 11.15am when suddenly everyone took cover on the floor.

She remembered: 'As I was under the table, I heard a couple of pops. And I looked at the rest of the girls under the table and I said, 'Did any of you hear that?'

'They all looked at me and brushed it off and said 'No, everything's fine. It was probably maybe five or 10 seconds that we were under the table before the janitor ran by and told everybody to just run.'

She climbed over a railing to get upstairs – telling DailyMail.com that her escape was made harder by the skirt and sandals she wore that day.

'I was surrounded by people,' she said. 'I felt completely alone and had no idea what to do or where to go.

'I ended up finding my best friend on the stairs and right as you go up the stairs, there was the choir room. And she said, 'we've got to go and tell my sister, she's in here.'

The students in the choir room had no idea what was going on but a teacher evacuated some students through a different door.

Laura and her friend took cover behind some chairs before realizing it was not enough.

'We ran into the choir office and several other students went in there - 30 to 40 other students.

'It was a very small office, there were no windows just a couple of desks and filing cabinets. We stuffed ourselves in like sardines and turned off the light.'

Sarah described to DailyMail.com the relief at seeing her sister Laura (far left) after the shooting, who she described as 'completely broken'

Laura (pictured with her family) ran her first marathon in 2006 – finding it both therapeutic and a way to get back in shape after having her kids, now three, five, seven and nine

Sarah (pictured with her family) ran her first marathon after her marriage then ran one between having each of her children, aged between four and 12

Some senior boys barricaded the door furniture and turned off the light. One girl had a cell phone and called police, who told her to turn it off and stay put.

'Before she turned off her cell phone, she called her mom,' Laura said. 'I remember it was quiet in that room and she called her mom and she said, ''Mom, I don't know what's happening but I don't think I'm ever going to see you again. And I just wanted to tell you that I loved you.''

'I was sitting under a little desk just sobbing. I sat under the desk and I prayed and prayed and prayed that somehow my sister was OK so that I didn't need to worry about that.'

Trapped in that room, they heard the shooters laughing and 'really trying to be obnoxious and loud.'

Right at my feet was one of my classmates who I had grown up with. His name was Danny Rohrbough. Laura Hill

Terrifyingly, they did step into the choir room but didn't try to get into the office where Laura and the other students were hiding.

And although the shooting subsided when the gunmen took their own lives, explosions continued to go off as they sat in the office.

Eventually, they heard banging on the door and realized it was a SWAT team. 'We were all so relieved.

'We moved everything out of the way and opened the door to find several SWAT team members in their uniforms pointing machine guns at our faces.

'Yelling at us, using terrible and scaring language and basically telling us that we were all suspects and that we had to do exactly what they said so that they could make sure we weren't in on it.'

The students were taken single file back through the cafeteria. They were taken out the door the shooters had entered the school through and were told to place their hands on their heads and run.

Tragically, Laura and her classmates were also forced to file past the bodies of those who hadn't survived.

'Right at my feet was one of my classmates who I had grown up with,' Laura told DailyMail.com. 'His name was Danny Rohrbough.

'He was laying dead on the ground and I had to walk over his body and run up the stairs where I found another body - Rachel Scott.

'I remember being really angry in those moments because I knew there had to be another way out of the school.

Her father was out of town for work and it took hours before Laura's mom, sister and grandfather were able to get to her.

'I hugged them and I don't think I let go of my mom for hours. I slept in her bed.'

Sarah described the relief at seeing her sister, who she described as 'completely broken.'

'Physically she was OK but you could tell she had witnessed some pretty terrible things and was just broken,' she said.

The sisters marked the 20th anniversary of Columbine by running the Boston Marathon together

The sisters, who both live in Utah with their families, say they are proud of how far they've through the trauma, attributing their progress to marathon running

Returning to school in the fall of 1999 was difficult for both sisters, despite teachers trying to change things to keep students calm.

'They changed the fire alarm because nobody could listen to that sound again without being triggered,' Sarah said.

Laura added: 'You just kind of had to do your best in your own way to make it through each day at school. I still was sent into a panic every time we had a fire alarm.

It took time but as time went by, both sisters have learned to live with what had happened - and use running as a coping mechanism.

Sarah quit soccer and joined the track team after the shooting. She ran her first marathon after her marriage then ran one between having each of her children, aged between four and 12.

Sister Laura followed and ran her first marathon in 2006 – finding it both therapeutic and a way to get back in shape after having her kids, now three, five, seven and nine.

'Up until that point I hadn't dealt with any of my post-traumatic stress or depression or anxiety or any of that stuff,' Laura told DailyMail.com.

'I had pushed it away and pretended like it wasn't there.'

Now the sisters have marked the 20th anniversary of Columbine by running the Boston Marathon together – and say it gave them the chance 'to celebrate how far we've come'.

Laura said: 'I felt every emotion under the sun. All of these past 20 years, the things that I have learned, I used in that marathon to make it through,

'It was the best possible way that I could have celebrated 20 years after the worst day we've ever experienced. It was incredible.'