At 10.28am local time on September 11, 2001, Lieutenant Mickey Kross of the New York Fire Department was in Stairwell B on the fourth floor of the World Trade Centre's north tower when it began to shake violently.
As the terrifying, metronomic thud of the upper floors collapsing grew louder and a downdraft became a whirlwind, he crouched and tried "to crawl into his helmet, like a turtle".
Ten years on, as a volunteer guide at Ground Zero, Kross, now 64, has become used to recounting that horrifying experience several times a month to groups of visitors.
It's a story that barely dulls with repetition: the scramble to get to the burning towers, climbing to the 23rd floor, helping a woman named Josephine, the collapse and then entombment, then following a shaft of light out.
When his audience finds out he is a survivor, they often ask "difficult questions", he says, as the partially constructed Freedom Tower looms over his shoulder. "I try to be pragmatic. I don't want to get religious on people, and I don't want to hurt anyone's feelings. If they want to believe God was looking out for me, that's fine. But what about looking out for the two and a half thousand other people?
"Of course, I don't say that ..."
For seven years following 9/11, Ground Zero lay cleared of ruins but unrestored, a 6.5ha arena for an epic bureaucratic struggle between developers, state and federal agencies, the governors of New York and New Jersey, insurers and the families of those killed. Now, flush with US$30 billion ($35.4 billion) of private and taxpayer funding, reconstruction is proceeding apace. Thousands of tourists mingle with construction workers on deadline to complete memorial pools on the footprint where the towers once stood.
The 540m One World Trade Centre, which has signed its first major tenant (Conde Nast, publisher of Vogue and Vanity Fair) can now, at 80 storeys, be seen from the city's approaches.
Not everyone is impressed. One World Trade Centre is set to be the most expensive office building in the US. At a time when there is a surfeit of unused commercial space, it's hardly needed, and through steep hikes in commuter tolls taxpayers are expected to underwrite US$13 billion of the costs.
In Manhattan, sensitivities over 9/11 persist. The support of the Mayor, Michael Bloomberg, for the construction of a Muslim interpretative centre three blocks from Ground Zero goes some way to explaining his poor standing in opinion polls.
Preparations for the anniversary commemorations are only mildly less contentious. Bloomberg has barred clergy from participating, and has discouraged politicians from touching on religious or political topics. "This cannot be political," Bloomberg says. The White House, too, has issued guidelines for the anniversary that stress "a positive, forward-looking narrative".
The guest list, too, is disputed: firefighters and emergency workers who responded on the morning of 9/11 have not been invited. "There's no room - construction is taking up the space," Kross explains. "I can't get too worked up, but some people are angry."
The approach of the 10th anniversary has placed new demands on him to recount his experience. "I knew this anniversary was going to be big, but it's worse than I thought," he said.
"My phone has been ringing off the hook. I've never seen such crowds; it's almost annoying. There are so many people you can't walk the streets."
Since commemorations began in earnest in June, Kross has been up to Maine to deliver a steel beam for a memorial. He has also been to an event in Shanksville, Pennsylvania, near where the fourth hijacked jet plunged into a field.
That, too, is now a tourist attraction. "It was a tragedy, but it's brought a lot of money and revitalised the place."
Before the attack, Kross was a keen student of history. His experience has focused his interest. "Almost everything I do is 9/11-related. I used to go to museums or take photographs, but I lost interest. This occupies my time, keeping up with the history, facts about the building, refreshing my mind about the official 9/11 commission report."
Visitors ask him what was going through his mind. "I tell them, 'I was just trying to keep my head clear enough to follow orders, and my orders were to get to a staging area on the 23rd floor and wait for further orders'."
But no further orders came. Kross and members of Engine 16 never heard the order to evacuate the building after the first tower fell. He was helping down Josephine when it crumbled.
It still perplexes Kross that he survived while the firefighter on the other side of Josephine, Sergeant Bacco, did not. "On about the 12th floor, with this woman between us, I reached over and said, 'Hi, I'm Mickey'. Next thing, we're inside, buried alive, and Bucca ends up dead outside. How did we separate? I can't make sense of it."
Josephine died two months ago and her body went unclaimed for five days in the morgue. "One of the fire chiefs got wind of this and arranged for a Fire Department funeral, and she got an honour guard, the fire truck, [former mayor Rudy] Giuliani came, the whole thing."
In the months after the attack, Kross virtually lived at the Red Cross centre used as a staging post for rescue workers. "There was camaraderie, a sense of purpose, that enabled us to do that," he recalls. "I couldn't understand why I felt so okay with it, because it's disgusting picking up fingers and feet and finding bodies wrapped into the steel."
His equanimity is remarkable. For a while, terrorism seemed to be stalking him: he was staying at the Royal National Hotel on Russell Square in the UK in July 2007 when extremists blew up a bus a few hundred feet away.
"Maybe I became inured to it, but I think I'm handling it pretty good. The 9/11 attack wasn't personal," he reasons.
Although he is immersed in the meaning of that shocking September morning, Kross's equable temperament provides a fascinating counterpoint to the televised solemnity this anniversary will convey. Every tour he takes, he says matter of factly, he concludes in the same way. "I always point them to the public bathrooms and ask them if they have any travel questions ..."