"For me it was just another day, another work day," white-haired former fire commissioner Thomas Von Essen told AFP in an interview in Paris.
"I was on my way to the office (and) got a call about a small plane crashing into the North Tower. I was very close to it, I got there quickly."
"Immediately we knew it wasn't a small plane, we knew it was more than that. And then when we felt the vibration in the North Tower, we thought it was an explosion. Turned out it was a plane hitting the South Tower.
"Then we knew we were being attacked," said the broad-shouldered Brooklyn native.
He remembers when his colleague Ray Downey, a collapse expert, said to him as they stood in the North Tower: "Boss, these buildings can come down."
"I'll always remember his eyes, you know there was no fear, there was no fright that we needed to get out of here immediately because this was something that was imminent," he said.
Summoned by mayor Rudolph Giuliani to join him in the crisis centre, Von Essen would never see Downey again.
The fire chief left Ground Zero where hundreds of his men had already begun climbing stairwells inside the doomed twin towers, each one 110-storeys high, to treat the injured and evacuate workers.
A total of 343 firefighters would disappear forever in the towers' rubble.
Then began the funerals and the letters of condolences, with Von Essen himself reading at least 60 eulogies.
"As bad as the day was, it got worse for a couple of months, just dealing with the effects of that day, the effects of a mother losing her husband and a son, or another captain who lost both boys," said Von Essen.
"It just went on and on. Such horrible losses. Separate from the tragedy of the day, it was dealing with the aftermath that was the hardest part for me."
Hit hard by the tragedy, Von Essen retired after 31 years at the New York Fire Department. He set up a consultancy with Giuliani before becoming a spokesman-security consultant for a large American company.
"I could have stayed as fire commissioner after that but I had to go, I was emotionally spent and I wanted to move on," the Brooklyn-born veteran firefighter said.
Reflecting on what has changed since that fateful day, Von Essen said: "Our federal agencies are more prepared. They are doing a better job at coordinating".
"I don't think that any country would do more than what we did for the victims. The authorities made a sincere effort to compensate and help everybody who was directly affected by the event."
"But we made a big mistake with the Iraq war (in 2003), even if we meant well. The lasting effect might be a good thing, it might lead to a democratic Iraq, but we don't know."
"If I look back, I'd say France was right about Iraq, we were wrong. It doesn't mean I think my country is bad".
Von Essen has little time for conspiracy theorists who say that the attacks were the work of president George W. Bush's administration seeking to justify invasions of Afghanistan and Iraq.
"At first, it was upsetting me," he said. "It is insulting for the people who lost loved ones. But then I realized, there is an enormous amount of morons in this world who don't know what to do with themselves and with computers they are even more dangerous, there is nothing you can do about it."
Von Essen still lives in New York but avoids returning to Ground Zero. Even the US military's killing of 9/11 mastermind Osama bin Laden in Pakistan in May gave him no pleasure beyond knowing he had been buried at sea.
"I didn't feel the excitement others felt. I got no joy out of it. I was just glad that the fish would eat him. I just wish this whole thing never happened."