This article, The evolution of the 9/11 conspiracy theories, originally appeared on the BBC News website.
It may be 10 years since the attacks in the US on 11 September, but conspiracy theories have not faded over time, says Mike Rudin, the host of BBC's "Conspiracy Files."
Numerous official reports have been published since the Twin Towers fell, but just when a piece of evidence casts doubt on one theory, the focus then shifts to the next "unanswered question".
Here are five of the most prominent 9/11 conspiracy theories circulating in online communities.
1. Failure to intercept the hijacked planes
The question: Why did the world's most powerful air force fail to intercept any of the four hijacked planes?
Conspiracy theorists say: The then US Vice President Dick Cheney ordered the military to stand down and not to intercept the planes.
Official reports say: This was a highly unusual multiple hijacking with violence on board, and where the transponder, which provides the exact location of the plane, was turned off or changed.
What is more, a routine military training exercise happened to be taking place that day at US air defence command.
Air traffic controller Colin Scoggins was in constant contact with the military and did not see any lack of response. There was confusion and a lack of communication between the civilian air traffic control (FAA) and the military.
The military's equipment was also outdated and designed to look out over the ocean to deal with a Cold War threat.
2. Collapse of the Twin Towers
The question: Why did the Twin Towers collapse so quickly, within their own footprint, after fires on a few floors that lasted only for an hour or two?
Conspiracy theorists say: The Twin Towers were destroyed by controlled demolitions. Theories relate to the rapid collapse (about 10 seconds), the relatively short-lived fires (56 minutes in World Trade Center 2 or 102 minutes in World Trade Center 1), reports of the sounds of explosions shortly before the collapse, and the violent ejections that could be seen at some windows many floors below the collapse.
Official reports say: An extensive inquiry by the National Institute of Standards and Technology concluded that the planes severed and damaged support columns and dislodged fire-proofing.
Around 10,000 gallons of jet fuel were spewed over many floors starting widespread fires. Temperatures of up to 1,000C caused the floors to sag and the perimeter columns to bend, causing the sounds of "explosions".
The massive weight of the floors dropped, creating a dynamic load far in excess of what the columns were designed for. Debris was forced out of the windows as the floors above collapsed.
Controlled demolition is always carried out from the bottom floors up, yet this collapse started at the top.
No evidence has ever been found of explosive charges despite the extensive hand searches and there is no evidence of any pre-cutting of columns or walls, which is routinely carried out in a controlled demolition.
3. Attack on the Pentagon
The question: How could an amateur pilot fly a commercial airplane in a complicated manoeuvre and crash it into the headquarters of the world's most powerful military, 78 minutes after the first report of a possible hijack and leave no trace?
Conspiracy theorists say: A commercial Boeing 757 did not hit the building but instead a missile, a small aircraft or an unmanned drone was used. But since evidence has increasingly shown that the American Airlines Flight 77 did hit the building, the emphasis has shifted to questioning the difficult approach manoeuvre. It is argued it was not under the control of al-Qaeda but the Pentagon itself.
Official reports say: Airplane wreckage, including the black boxes, were recovered from the scene and they were catalogued by the FBI.
Although some early video did not show much wreckage, there is a good deal of video and still photography which shows airplane wreckage and evidence of the flight path, such as broken lamp posts.
The remains of crew and passengers on the plane were found and positively identified by DNA. Witnesses also saw the airplane strike the Pentagon.
4. The fourth plane - United Airlines flight 93
The question: Why was the crash site at Shanksville, Pennsylvania so small and why was the aircraft debris not visible?
Conspiracy theorists argue: United Airlines flight 93 was shot down by a missile and disintegrated in mid air, scattering the wreckage over a large area.
Official reports say: There are clear photographs showing aircraft wreckage and the cockpit voice recorder, which showed there had been a passenger revolt and the hijackers had deliberately crashed the plane.
Initial theories that heavy debris was scattered many miles from the main crash site turned out to be false. In fact the wind had blown light debris such as paper and insulation just over a mile.
Another theory was based on a misquote from the local coroner, Wally Miller, who said he stopped being a coroner after about 20 minutes because there were no bodies. What he also said was that he quickly realised it was a plane crash and there would have to be a large funeral service for the many victims.
In addition, the military never gave orders to the air force to shoot the commercial airliner down.
5. Collapse of World Trade Center Building 7
The question: How could a skyscraper, which was not hit by a plane, collapse so quickly and symmetrically, when no other steel framed skyscraper has collapsed because of fire?
Conspiracy theorists say: The World Trade Center Building 7 was destroyed by a controlled demolition using both explosives and incendiaries.
Initially the focus was on the phrase "pull it" used by the owner, Larry Silverstein, in a TV interview. But in fact he was talking about pulling firefighters back. (Demolition experts do not use the term "pull it" as slang for setting off explosives.)
Now the focus has shifted to the speed of the collapse which reached near free fall for 2.25 seconds. It is argued only explosives could make it collapse so quickly and symmetrically.
[ Photos: The World Trade Center towers are attacked ]
Some scientists, who are sceptical of the official account, have examined four dust samples from Ground Zero and claim to have found thermitic material which reacts violently when heated up. They claim tonnes of thermite and conventional explosives were rigged inside, not just WTC7, but also the Twin Towers.
Official reports say: A three-year investigation by the National Institute of Standards and Technology concluded that the building collapsed because of uncontrolled fires, started by the collapse of the nearby North Tower, and which burnt for seven hours.
The mains water feeding the emergency sprinkler system was severed. No evidence has ever been found of explosive charges and there are no recordings of a series of very loud explosions that would have been expected with controlled demolition.
Furthermore, there is an alternative explanation for the "thermitic material" the sceptical scientists found in the dust - it is just a type of primer paint. It's calculated 1,200,000 tonnes of building materials were pulverised at the World Trade Center and most minerals are present in the dust (not necessarily in a large quantity). More extensive sampling of the dust has not found any evidence of thermite or explosives, says a report from the US Geological Survey and another from RJ Lee.
Why the 9/11 conspiracies have changed
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.
Gfk NOP carried out the opinion poll for the BBC in the UK and USA in July. Both were telephone polls with 1000 adults and the margin of error is +/- 3%
Question: Attacks were made on the World Trade Centre and the Pentagon on September 11th 2001, commonly known as 9/11. It is generally accepted that these attacks were carried out by 'Al Qaeda'. However some people have suggested there was a wider conspiracy that included the American Government. Do you, yourself, believe that there was a wider conspiracy, or not?
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