WARNING: CONTAINS DISTRESSING CONTENT
It's been 18 years since we watched on in horror as New York was struck by the single deadliest terror attack the world has ever seen, causing the deaths of 3,000 people and injuring some 6,000 more.
The attack on 11 September 2001 saw a group of Al Qaeda terrorists hijack four planes, two of which were crashed into the North and South towers of the World Trade Center in New York City, while another was flown into the Pentagon in Washington DC and the fourth into a field in Pennsylvania.
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But with such a huge death toll, it's important to remember that there are thousands of names, faces and lives behind the statistics.
The final calls and messages to those stuck on hijacked planes or trapped inside the Twin Towers are a harrowing reminder not only of the people who lost their lives, but also the loved ones they knew they were leaving behind.
Melissa Doi was working on the 83rd floor of the South Tower.
In her last moments she made a 999 call, telling the operator: "There's no one here yet and the floor's completely engulfed. We're on the floor and we can't breathe...
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"It's very hot... All I see is smoke."
She added: "I'm going to die, aren't I?"
Doi tried to get the dispatcher to set up a three-way call so that she could she could speak to her mother one last time. Tragically, they were unable to do so.
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Melissa Harrington Hughes - who was only in New York for one day - made her last call to her husband, Sean, who was still in bed and didn't pick up due to the time difference.
She told him: "I just want to let you know I love you and I'm stuck in this building in New York.
"There's lot of smoke and I just wanted you to know that I love you always."
Fire chief Orio Palmer managed to help many trapped people escape to safety, leading a team up to the impact zone on the 78th floor. He did not return.
Speaking to his colleagues his radio, he said bravely: "I didn't hear fear, I didn't hear panic.
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"When the tape is made public to the world, people will hear that they all went about their jobs without fear, and selflessly."
Beverly Eckert knew her husband, Sean Rooney, would have been at work in the Twin Towers that day.
He called her, but was not able to give her the good news she needed.
Beverly told the New York magazine: "He told me he was on the 105th floor and I knew right away that Sean was never coming home.
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"After long minutes of talking, he whispered 'I love you' over and over. Then I heard this loud explosion."
At the time, Sean was still alive, but the noise made it clear to both of them that the tower was in the process of collapsing.
Beverly added: "I called his name in the phone over and over. Then I just sat there huddled on the floor holding the phone to my heart."
Meanwhile, CeeCee Lyles was one of the flight attendants on Flight 93 - the only plane not to hit its target.
She tried twice to call her husband, a police officer, but as he'd just come off a night shift, he did not hear the phone ring.
Instead, she left him a message, telling him: "Hi baby, baby you have to listen to me carefully. I'm on a plane that's been hijacked, I'm calling from the plane.
"I want to tell you that I love you. Please tell my children that I love them very much and I'm so sorry baby.
"I don't know what to say. There's three guys, they've hijacked the plane, we're turned around and I heard that there's planes that have been flown into the World Trade Centre.
"I hope to see your face again, baby. I love you."
Betty Ong was a flight attendant on the first plane to be flown into the Twin Towers.
In her final moments, she managed to call the authorities, providing vital information about what had happened - potentially saving countless lives by helping officials identify the attackers, and ground dozens more flights.
Our thoughts go out to all those who lost loved ones in the 9/11 attack, or have been in any way affected by the horrific event.