Learn more about why I came to write the novel and my take on the actual events:

Why did you write about 9/11?

9/11 was the most horrifying event I’ve ever witnessed. And while it’s instinctive to want to do something to help, I was sitting in an office in London at the time, watching the events on TV with a crowd of colleagues, all as shocked as I was. And part of the horror was the feeling of sheer helplessness that I, and hundreds of millions of others, felt.

Then on July 7, 2005, I came very close—I’ll never know exactly how close—to being in London’s own Ground Zero. What happened is that I was in the front carriage of a Piccadilly Line train heading southwards into London, but just before 8.45am, for no particular reason, I decided to transfer to a different line. Minutes later, a terrorist detonated a bomb in the front carriage of either my original train, or the train one or two trains ahead of it, at the precise part of the train where I had been standing. So now I couldn’t help thinking, how would things have turned out if it had been my train and I’d still been on it?

Somehow this evolved into a recurring 2am obsession of trying to figure out how I, or anyone else, could have escaped the World Trade Center. And when I researched the topic, I was appalled at how poor the processes for evacuation were, and how badly they were applied on the day. Finally, I realised that the only way I’d be able to stop thinking about was to write it all down.

I particularly hope that my novel helps debunk the enduring conspiracy theory that the two tower collapses were triggered by a series of controlled explosions just below the points of impact.

How accurate is your description of the South Tower and the events of 9/11?

My research included hundreds of first-hand accounts, tens of hours of video footage, reconstructions and re-enactments, architectural blueprints, computer simulations, and scores of technical descriptions of the towers, their stairwells and elevators. Unfortunately I wasn’t able to find every technical detail I would have liked to and I had to make a few informed guesses. There were also occasions where I found contradictory information and had to make a choice between the two. I apologise for any inaccuracies.

Are any of the characters based on real people?

I did my best to ensure that the people in my book can’t be equated with real 9/11 victims. That wouldn’t be fair to their memories or to their family members. However, I couldn’t not include a FDNY officer—and he’ll inevitably be equated with one of the stand-out heroes of the day, Battalion Chief Orio Palmer, who did indeed make it up to the 78^th floor Sky Lobby. Hopefully his fictional counterpart does him justice.

Is it right to portray some characters as villains?

Under extreme stress, some people behave better. Others behave worse. To feel authentic, I decided that my novel had to reflect that.

I can understand why we’d like to believe that every one of the thousands of people in the towers on that day was a hero. Yet we know that’s an unrealistic assumption. At capitalism’s cutting edge, we know there are plenty of extreme personalities. It wouldn’t ring true to place such people under such stress and expect everyone to behave impeccably.

And there is evidence for my decision: A woman in the Millennium Hilton Hotel, across the road from the towers, reported how people were “fighting” to force their way into her packed elevator after the South Tower was hit. Someone else reports that while evacuating down the stairs of the South Tower, a man was trying to shove people aside to get down faster. And while I’ve not found real evidence for this, other reports suggest that some of the people who fell out of the towers were in fact pushed by others behind them, desperate for air. If you watch this video clip (from 2:32) that shows people falling out of a North Tower window in quick succession, it’s easy to imagine that there might have been some pushing going on.

[Spoilers follow]

The South Tower suggests that it was the explosive reaction of particular chemical elements that triggered the collapses of the towers. Is that based on fact or dramatic license?

A number of firefighters reported hearing a series of explosions right before the North Tower collapsed, and there’s also visual evidence of this on at least one video—showing bursts of flame from just below the point of impact as the tower begins to collapse. (Another video shows violent gusts of air coming from windows on one of the towers, before propelling what might be a person out of a window with tremendous force.)

Conspiracy theorists leapt on these reports, claiming that they were evidence that the towers’ collapse was in fact a controlled demolition, caused by pre-installed explosives.

The official report completely discounted any role for the planes’ aluminum fuselages and wings, claiming that they would have been completely shredded in the impacts. However, scientists including Frank Greening and Christian Simensen maintain that if bigger chunks of aluminum had stayed together, all the other ingredients for the explosions were present. (And one South Tower survivor, Stanley Praimnath, whose office was on the 81st floor, recalled seeing a large piece of aluminum buried in his ruined door after the impact. Unfortunately, in a gift to the conspiracy theorists, the Port Authority has denied Simensen access to WTC debris to test his theory.

You can watch a short video explaining the science here.

Were there really places where people could escape the worst of the smoke?

Yes. According to this extremely well-researched New York Times article, there were interior conference rooms in both towers that were relatively smoke-free the whole time.

How feasible are the more exotic escape attempts that Nick tries out in the story?

The only method that’s been successfully attempted is the one using the window-cleaning tracks—George Willig successfully used them to climb the South Tower with his own custom-designed equipment in 1977.

A parachutist named John Vincent base jumped from the South Tower’s window cleaning gondola in 1991, so no theoretical problem there, but I doubt whether he’d be comfortable trying it with Nick’s improvised parachutes. I have to say I’m proud of my ‘rotor-chute’ innovation and in theory, a professionally designed version of the right size and materials should work, but I haven’t found any reference to such a thing. Maybe I should patent it. (I’d prefer that someone else tested it though.)

As far as the door bombs go, all the chemical reactions I’ve described are real—or so my sources claim—but I can’t find any evidence of how they’d work on the door or wall in question. Bicycle thieves have been using the ‘cold spray aerosol’ technique for decades.

If you watch action movies, you’d assume that climbing down an elevator shaft is a piece of cake. In reality, trying to cling on to a greasy cable would take some clever knots and plenty of time and training.

Abseiling with electric cable and phone cords has to be theoretically possible, but I must admit I haven’t actually tested it for myself.

Balloons? Everyone knows about the guy who took a flight in a deckchair using a fleet of helium balloons, but Nick’s technique is a bit different. I suspect that real world results would be pretty much as I’ve described in the book.

Could you have simply chimney-climbed down the tower between two columns? Again, it would have been theoretically possible, although the columns were closer together than you’d ideally have liked—and they weren’t wide enough to get your hands nicely spread out in front of you. At least one person did attempt this feat on 9/11 but failed.

Was a helicopter rescue feasible?

When Al Qaeda attacked the WTC in 1993, NYPD Sgt. Timothy Farrell rappelled down to the North Tower roof from a helicopter, broke through a locked rooftop door and went to work, getting people up to the roof and helping to rescue others from elevators. Over the next few hours, NYPD helicopters airlifted 28 people from the North Tower.

Since then, the Port Authority took the decision that rooftop evacuation was too risky and decided to keep the rooftop doors locked, for a number of reasons. One, the rooftops were packed with structures, fencing, cabling and antennas. Two, it’s almost always safer to go down rather than up when there’s a fire (fire usually goes up). Three, helicopter rescue is slow—and a mass evacuation could easily turn into a panic.

On 9/11, there were more reasons why helicopter evacuation was hazardous. The whole of the South Tower’s roof, and all but a tiny part of the North Tower’s roof, were obliterated by dense smoke. A landing was simply impossible, and flying through the smoke could have caused engine failure. In addition, the combination of a breeze and rapidly rising hot air would have created very dangerous flying conditions. For any normal helicopter and any normal pilot, attempting a rescue was out of the question.

However, Chinook helicopters are a different proposition. At over 24,000 pounds when empty, they don’t get tossed around too easily by gusts. Their twin rotors make them extraordinarily maneuverable. Their enormous tail flaps make an actual landing unnecessary: they can actually hover over water while a boat drives right up the flap—so they wouldn’t have had to fly through the smoke and the worst of the hot air on the day: they’d just have to lower their tail flaps to the northwest corner of the roof. And their military pilots are a breed apart: they’re trained to drop and extract soldiers from crazy terrain, in crazy weather, under fire. If anyone could have done it, they’d be the ones.

So it comes down to a race against time. Could a Chinook have reached the towers in time to rescue anyone? The closest Air Force bases to Manhattan are at Otis Air National Guard Base at Cape Cod, Massachusetts, and at Langley Air Force Base, Virginia. (There were no aircraft carriers close to New York on the day.) So assuming that there were Chinooks available at Otis or Langley, they would have had a 200-mile-plus trip to the towers. With a maximum speed of 196mph, they were effectively over an hour away.

But we also need to factor in time for someone to authorise the mission, plus time to round up a crew and equipment. On the morning of 9/11, there happened to be F-15 fighters in the air near Long Island, and at 9:13, they were ordered to head for Manhattan. Let’s imagine that Chinooks could have been authorised at the same time. If we imagine that they could have been airborne, with crew and equipment, within half an hour, and took an hour to reach the WTC, they would still almost certainly have arrived after 10:28, when the North Tower collapsed. Even if they’d arrived earlier, it would still have taken time to cut through the security fencing around the edge of the rooftop, break in through a door and get any survivors out. A hopeless undertaking: but given more time, who knows?

It’s likely that there were commercially operated Chinooks closer to the scene, but would their pilots have been skilled enough to manage the very tricky conditions? Would they have had the equipment necessary to get through the fencing and break through the door? And would they have been allowed to approach that closely anyway?

What’s your opinion of the 9/11 conspiracy theory—that is was all planned and executed by the US government/CIA?

The most impressive thing about 9/11 conspiracy theories is just how many conflicting versions there are. In fact, the only consensus there is among the so-called “truthers” is that there was some kind of conspiracy, with the aim of prodding America into a war in the Middle East.

In support of these claims, masses of doctored footage and photographs appear on websites which then link backwards and forwards to others, allowing people to follow the supposed “evidence trails” for themselves.

Let’s start with the more bizarre theories:

• Either or both of the planes that crashed into the South Tower and the Pentagon was either a missile or a military plane

• At least one of the planes was a hologram

• One or more of the planes reportedly involved in 9/11 are still in service

• It was all done by aliens in a flying saucer

Unsurprisingly, none of this stands up. There’s more than enough footage of the WTC plane impacts and masses of eye-witnesses. Not one of the passengers in the planes was ever seen again, and quite a few managed to make pre-impact phone calls from the planes reporting the hijacks and their fears about the outcomes. While the footage of the plane crashing into the Pentagon is extremely brief and low-quality, there were still plenty of eye-witnesses.

A more conventional conspiracy theory is that the planes were hijacked by CIA operatives, not Al Qaeda terrorists. The “proof” of this is the claim that an Al Qaeda terrorist who was reported to have been on one of the planes phoned his father later on to say he had not been on one of the planes and was in fact alive and well. But without a recording of the call and other prior voice recordings of the alleged terrorist to compare it with, this claim is nothing but hearsay.

One anomaly is the passport belonging to one of the terrorists that was found somewhere below the towers before they collapsed. The conclusion the “truthers” draw from this is that it must have been planted by the CIA to “prove” Al Qaeda’s involvement. Unfortunately, the man who handed it over to a police officer ran away immediately afterwards (the scene was of general panic, remember) so nobody knows exactly where it was found. But while it seems improbable that it survived intact (merely sodden with jet fuel ejected from the towers) it’s far from impossible.

Another argument the “truthers” use is the unlikely competence of the pilots—they argue that it’s simply impossible for an amateur to fly a Boeing 767 into a building with such precision. But there’s plenty of documentation proving that the terrorist pilots took an accelerated pilot program—obtaining their instrument ratings nearly a year before 9/11—and continued practising on a range of airline simulators including a Pan Am 767 simulator. And flying conditions on the day were pretty much ideal.

Next up is the theory that the CIA was somehow in league with Al Qaeda: while Al Qaeda’s task was to crash the planes, the CIA’s job was to make sure the buildings actually collapsed. There’s absolutely no proof of this, but what makes it not entirely unbelievable is that the Towers’ elevators underwent scheduled alterations just prior to 9/11, providing the opportunity for placement of “the explosives”.

Sadly, while the scheduled elevator work did take place, it seems that this was actually when the doors were altered to make them impossible to open manually. This “safety feature” had been debated for years and was finally authorised just in time to trap scores of people in malfunctioning elevators. Unfortunate timing but hardly evidence of a bomb plot.

But the “truthers” still contend that even if the planes were hijacked by Al Qaeda, the impacts and burning jet fuel were not destructive enough to bring the towers down in so little time—and therefore the towers must have been brought down by explosives. And while I agree that there were explosions, I don’t believe they were man-made.

There are four possible conspiracy-free reasons for the towers’ collapse:

1. The planes’ impact bent and severed enough of the vertical steel support columns—both at the entry points and in the core of the towers—to warp and overstress key load-bearing points. As each point sheared, more stress was placed on the following ones, finally resulting in the collapse of an entire floor which triggered a “house of cards” chain reaction.

2. The burning jet fuel gradually weakened the vertical steel support columns in the towers’ cores until they couldn’t bear the load any longer.

3. There was some kind of explosion—or a series of explosions—caused by an unfortunate combination of materials and heat.

4. A combination of two or more of the above, with the North Tower being further weakened by the South Tower’s collapse.

Examining these one by one…

1. There was obviously plenty of impact damage. You can clearly see the severed exterior columns. How many interior columns were severed will never be definitively decided, although subsequent computer modelling suggests many were. Witnesses report that they heard the building buckling. And if the columns weren’t just severed but bent, that could feasibly put undesigned-for leverage on key joins. A support point for this theory is that the South Tower was first to fall—and Flight 175 was moving far faster than Flight 11 at the moment of impact.

2. Heat-induced weakening of the interior columns in such a short time frame is difficult to believe. The columns were well insulated and shouldn’t have lost strength so quickly. Some experts have suggested that it would have taken something like nine hours—which is pretty much how long the burning WTC 9 took to collapse. However, we don’t know how badly the insulation was damaged by the planes, and this could explain why the South and North Towers collapsed well before Tower 9.

3. Molten aluminum is extremely dangerous stuff, and there had been some tragic smelting accidents before the science was understood. Basically, a mixture of molten aluminum and water (plus iron oxide or gypsum as a catalyst) is about four times more explosive than TNT. There was plenty of aluminum (from the planes), more than enough heat to melt it (initially from the burning jet fuel, but quickly followed by burning carpets, furniture, etcetera), plenty of water from the severed water pipes (survivors reported seeing water flowing down the stairs) and plenty of gypsum from all the internal sheetrock walls. There is footage that shows what looks like molten metal pouring out of a tower at some point. There is also footage showing what looks like minor explosions before the eventual collapses—one of them propelling a man right out of a window. So this looks like a likely contender. Critics of this theory say that the aluminum would have been too finely shredded to form pools of liquid, but there’s no evidence for this—and South Tower survivor Stanley Praimnath reported seeing a large chunk of aluminum stuck in his office door. Critics also say that the collapses look too ‘neat’ to have been caused by a messy aluminum explosion—but they tend to quote made-up eye-witness reports and incorrect collapse-times.

In the fractious and highly political world of the American intelligence services, it’s extremely implausible that such a treasonous conspiracy could have been planned and perpetrated without anyone leaking the details along the way. The conspirators would also have had to conceal any bomb-planting activity from the WTC’s chief of security, the ex-FBI John O’Neill, a man deeply suspicious of both Al Qaeda and the CIA. You might hypothesize that he was part of the conspiracy—but you have to remember that he was in his office in the North Tower on 9/11 and died when the tower collapsed. (And before anyone suggests that he merely ‘disappeared’, his remains were actually recovered.)

It would be pure optimism to imagine that the conspiracy theories will go away: there are far too many contradictory eye-witness reports, unexplainable on-air TV comments and other anomalies. But then, even very simple events generate highly contradictory eye-witness reports, and the most mundane sports event inspires plenty of illogical commentary, so that’s not surprising.

It’s also depressing how filmmakers continue to interview “experts” who passionately tout their theories, without exposing them to the harsh light of debate.

As I suggest in the book, one way to settle the exact causes of the collapses would be to run a series of massively detailed simulations… which is where Nick’s recurrent resets seem to fit in.