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Frank Culbertson was the only American off the planet during the attacks on 11 September 2001, having been inside the International Space Station with two Russian cosmonauts at the time, around 250 miles (400km) above Earth.
Looking down from the space station, Culbertson saw smoke billowing from New York, which was coming from the second of the Twin Towers.
Culbertson spoke about the harrowing moment in a 2014 documentary called Astronauts: Houston We Have a Problem, a clip from which has recently been shared on TikTok by @footage_of_the_wtc.
In the clip, he recalls: "I raced around and found a video camera and a window facing in the right direction.
"The weather was perfectly clear that day. I could easily see New York City - a big black column of smoke coming out of the city, and as I zoomed in with the video camera, I could see this big grey blob enveloping southern Manhattan.
"What we were seeing was the second tower coming down."
Culbertson was the sole American astronaut on the third mission to man the ISS, which was still under construction at the time.
He only found out what was going on when he called Earth for a routine medical check-up, when he was told: "Frank, we're not having a very good day down here."
In the clip, we also hear audio of Frank in the space station, telling Mission Control: "We can see New York City, and the smoke from the fires.
"Our prayers and thoughts go out to all the people there and everywhere else.
"Here I'm looking up and down the East Coast to see if I can see anything else."
Later in the audio clip, he adds: "I just want the folks in New York to know that their city still looks very beautiful from space."
In the documentary, Frank explains how NASA arranged a phone call with his wife later that day so he could check on his family.
He says: "It is an isolated feeling to know that you can't be with your loved ones to make sure they're safe.
"NASA did arrange a phone call with my wife that evening. It felt good to talk to her to know she was okay and the kids were okay."
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