Conspiracy theories have proliferated following the attacks in the US on 11 September 2001, and over the last decade these theories have taken many twists and turns, explains Mike Rudin.
Ten years on from the attacks which killed nearly 3,000 people, conspiracy theories have continued to evolve. They now question every aspect of the official account, despite the fact that every year has provided more witnesses and evidence to bolster the official explanation.
An opinion poll, carried out by Gfk NOP for BBC's The Conspiracy Files in 2011, found that 14% of people questioned in the UK and 15% in the US did not believe the official explanation that al-Qaeda was responsible, and instead believed the US government was involved in a wider conspiracy. Among 16 to 24-year-olds that belief rises to around one in four.
Since 9/11 there have been numerous lengthy and painstaking official reports - the 9/11 Commission, congressional investigations and many inquiries by the National Institute of Standards and Technology. None has ever found any evidence of a wider conspiracy.
The myriad of conspiracy theories, on the other hand, are rarely spelt out in great detail - perhaps because when they are, they have been quickly debunked. Nor is a motive usually explained.
The starting point for 9/11 conspiracies is that many people find it hard to believe 19 young men, armed with just knives and box-cutters, could casually walk through airport security, hijack four commercial planes and then within the space of 77 minutes destroy three of the iconic symbols of America's power, in the face of the world's most powerful and technologically-advanced military superpower.
It is a shocking thought.
As with many conspiracy theories there is a distrust of anything official and disbelief that government and security forces, which are so often portrayed as invincible, can be beaten by a small group of poorly-armed men.
It is a similar argument that questions whether a lone gunman could have killed President John F Kennedy, then the most powerful and best-protected man on the earth, or how someone so special as Princess Diana could die in a car crash.
Nothing is taken on trust about 9/11. If an eyewitness, an official or an expert counters a conspiracy theory, their motives are immediately questioned.
And the theories are ever evolving. When evidence comes forward that casts doubt on a theory, one rarely hears an admission that the theory must be wrong. Instead the focus shifts to the latest "unanswered question".
"We don't know the full story of exactly what happened," says American radio talk show host Alex Jones. "We know the official story is completely unproven and a fairy tale. I'm saying that it needs to be investigated."
A number of conspiracies focus on the actual collapse of the Twin Towers at the World Trade Center.
Initially many questioned how such huge skyscrapers, which had dominated the Manhattan skyline for so long, could be brought down by an hour or so of fires - alluding to the possibility of some kind of controlled demolition.
But then the official report set out a rational explanation. And it also pointed out that contrary to the conspiracy theory, controlled demolition is always bottom up and not the top down collapse of the Twin Towers.
So then the focus shifted to World Trade Center Building 7 - another huge skyscraper which also collapsed on 9/11, but which was not hit by a plane.
The theory is that tonnes of explosives and an incendiary called thermite were used in a controlled demolition to destroy the building from the bottom up.
But when it is pointed out that thermite has never been used in controlled demolition, the theory once again moves on and claims that new and secret types of explosives and incendiaries were used.
So what is the attraction of conspiracy theories? And why are they so persistent?
Writer and producer of 90s US television series The X-Files, Frank Spotnitz, offers an explanation. He argues that we live in an age of anxiety, where we do not know who to trust and what to believe in. Conspiracy theories, he says, offer "a magic key that fits all the pieces together" and makes sense of our uncertain world.
Other conspiracy theories question whether a commercial Boeing 757 even hit the US military's Pentagon headquarters in Washington DC.
And another suggests the fourth plane, which crashed in a field in Shanksville, Pennsylvania, was shot down by a American military missile.
Even the death of Osama Bin Laden in May 2011 is questioned. A host of different conspiracy theories suggest he died as early as 2001 or even that he was captured by American forces some time later.
"It is utterly astonishing that we should be able to kill a man who actually died nine years earlier in this fantasy event in Pakistan," says Prof Jim Fetzer.
But judging by the BBC's opinion poll, belief in conspiracy theories about 9/11 seems set to continue for a long time to come.