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Twenty years ago, Andrew Cullen felt the explosion of a plane hitting the skyscraper next to him just as much as he heard it.

Instinct led him to a series of actions in the following minutes which would turn out to be critical.

A split-second decision over which lift button to push turned out to be the biggest decision of his life.

Hitting down instead of up just moments before the second plane hit the south tower of the World Trade Center on 11 September 2001 ensured the Scot would be alive to re-tell his story two decades on.

Born in Motherwell, North Lanarkshire, Andrew was 10 when his dad got a job in Pittsburgh and moved the family to the US in 1980.

At the age of 25, he moved to Manhattan to work as an analyst at investment firm Keefe, Bruyette & Woods (KBR).

On 11 September, 2001, he arrived at work on the 89th floor of the South Tower. He was part of a team working on a big deal for a bank.

It was a beautiful day.

Reliving the morning on ����ý Radio's Drivetime programme, Andrew said: "It was very clear, there wasn't a cloud in the sky, so from the 89th floor you could see forever, straight over both the Statue of Liberty as well as east over to Queens and Brooklyn.

"Usually at about 06:45, the traders and the analysts and the other sales guys start rolling in and it was really while the salesmen and traders were making their morning phone calls that we felt and heard an enormous explosion from the North Tower.

"We had no idea what it was, but the ferocity of the explosion caused our building to sort of whipsaw and being on the south east side of the building I was able to see some of the debris coming out of the side of the building."

He said: "I got out of my chair and started walking north along the trading desk, and arrived at the equity desk where Derek Sword, my colleague from Dundee was sitting. I believe he was on the phone with his brother Alan at the time trying to surmise what had happened.

"At that point myself and three colleagues made the decision to get out of the building so we headed towards the stairwell and began descending from the 89th floor down to the Sky Lobby at the 44th floor and it was at that point that the port authority instructed us to go back to our offices, so they sort of shuffled us into one of the freight elevators.

"At that point the button for 78 was lit but we decided we wanted to go down so I was the one who pounded the lobby button just a few moments before the second plane hit the tower above us."

If the lift had gone up, they would have been killed.

He said: "At that point it was chaos. I got expelled from the elevator with some colleagues. I crawled around the corner to the stairwell and maybe 30 seconds later one of my colleagues came down to the 43rd floor and we made the decision we needed to get out of the building as quickly as possible."

Andrew and his colleagues still had no idea what had happened. The only logical explanation they had was that the other tower had collapsed into their tower. Now he understands that during the massive second explosion, the jet fuel came down the lift shaft and blew the lift apart.

He said: "I am not sure how many survivors there were in the elevator but I was told it had dropped at that point. So we were fairly fortunate to be on the front."

After finding a stairwell, they managed to get to the lobby where port authority officers and fire crews were trying to organise people to get them out of the building.

They did not want to waste a minute so they found the east side of the building and headed into the courtyard and then over towards the street.

"Once we had seen the effect of the planes flying into the building and witnessed some people jumping out of the building, we headed to a health club we knew to get away from the scene," Andrew said. "We had only arrived there for about 10 minutes before our tower collapsed and sent the debris down Wall Street where a lot of the store fronts were blown in including the Equinox club where we were.

"We were fairly fortunate that we didn't get hit with shrapnel and other debris that was flying around. We got knocked down to the floor and we were behind a desk when it blew in over the top of us. Once the dust had settled we made our way into one of the bathrooms and grabbed wet towels and wrapped them around our faces to avoid inhaling smoke.

"Again we found the east of the building and made our way over to the East River but it was feeling our way along because it was like walking through a blackout - that's how thick the dust and debris was. When we hit the East River we found about a million other people.

Andrew believes it was instinct that got him out of the tower in time.

"When you have a threat that faces you and you don't know what it is, you have a choice," he said. "You either face it or you get the hell out of the situation.

"All I knew was that the explosion I felt from the first tower being hit was extremely powerful, and I didn't need to be a third of a mile up in the air. I am deathly terrified of heights so that ironically might have saved my life."

Later in the day Andrew helped work the phones with other employees who survived and by the end of the day they had an idea of who got out and who didn't.

Derek, his Dundee friend, was one of the 2,606 who lost their lives in the Twin Towers attack.

The pair bonded over their Scottish roots on Andrew's first day in the job. They were introduced by one of KBW's CEOs John Berry who also died. The firm lost 67 employees that day.

The surviving staff spent the rest of the week in and out of New York hospitals checking logs to see if any of their colleagues had survived.

Andrew, now 51, is a history and economics teacher in South Carolina. He has attended a few memorial services since that day but prefers to keep a low profile.

"You manage it but you don't get over it, " he said. "It's a process. It is always there and it might change your life but it shouldn't stop you living it.

"It forced me to think about how I was living my life and change my path a bit, rather than following what I thought was a prescriptive path, doing things I was a bit more interested in, maybe being a bit more impetuous than I would have been."