Rare news footage of the second plane hitting the World Trade Center on 11 September, 2001, has been unearthed online.
A CNN clip, originally broadcast on the day of the 9/11 attacks, was shared on YouTube 12 years ago, but some viewers are only just discovering it for the first time.
On the morning of 11 September, four planes were hijacked by the al-Qaeda terror group.
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Two were flown into the Twin Towers in New York City, while another hit the Pentagon and a fourth - intended for either the White House or the US Capitol Building - was crashed into a field in Pennsylvania.
The attacks killed almost 3,000 people, meanwhile, thousands more were injured.
The devastating clip shows the World Trade Center surrounded by plumes of black smoke, following the impact of this first plane - Flight 11 - into the North Tower.
Moments later, a second plane - Flight 175 - can be seen flying directly into the South Tower, hitting floors 77 to 85.
Within a period of 17 minutes, both towers had been struck, meanwhile crowds on the street could be heard crying and screaming at what they were witnessing.
"That angle really shows the speed upon impact, and the roar of the jet engine really freaked people out," one person wrote.
While another commented: "The moment we knew this wasn't an accident. This was the real end of the 90s. The world was never the same. Rest In Peace."
And a third said: "This used to be the most showed angle of the second plane hitting but now I have to search so hard just to find this basic clip."
Meanwhile, a fourth wrote: "This for me is one of the most symbolic pieces of footage from 9/11. It shows how deliberate and evil the spectacle was, but the screams and horror from the ground made it the most chilling."
Others said the footage reminds them just how real the attack was, with another adding: "Every single time I watch footage from this day, I have to remind myself that it's real. That it's not some poorly made action movie.
"This actually happened. 20 years later and I still cry. I was only 11 when it happened so I didn't understand what was happening but now I understand better than ever. I wish I'd been older then so I could've understood it better in real time."
Lucy Devine