Culbertson was approximately 250 miles (400km) above Earth when he first saw what would have been one of the most devastating events in America right before his eyes.
When looking down from the space station, he noticed a gust of smoke rising in New York, specifically, from the second of the Twin Towers.
In a 2014 documentary titled Astronauts: Houston We Have a Problem, Culbertson opened up about that harrowing moment.
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."
Hauntingly, he then revealed: "What we were seeing was the second tower coming down."
Culbertson was the sole American astronaut on board the International Space Station, which was still under construction at the time.
Upon calling Earth for a routine medical check-up, Culbertson soon realised the gravity of what was taking place below him.
When asked what was going on, he was told: "Frank, we're not having a very good day down here."
In the footage, his personal audio is also heard as he reacts to the attacks when he tells 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."
Tragically, Culbertson also adds: "I just want the folks in New York to know that their city still looks very beautiful from space."
In the Astronauts: Houston We Have a Problem documentary, he also explains how NASA set up a phone call between him and his wife later that day so he could check on his family during the devastating attacks.
"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," he reported.
The events of 9/11 tragically saw almost 3,000 people lose their lives when 19 hijackers from the terror group Al-Qaeda took control of four planes.
More than 6,000 others were gravely injured.Featured Image Credit: NASA