Lots of questions have been raised about 9/11:
Why was the hole in the Pentagon wall so small and why was there so little damage to the outer wall?
Why does it look like there is no plane at the crash site in Pennsylvania where flight United 93 came down?
Why did a building called World Trade Centre Building 7 collapse even though it was never hit by an aircraft?
And why was America so unprepared when terror attack warnings had been received?
Through the internet and the media generally, allegations of complicity by the US government in the 9/11 attacks are intensifying.
We’ve talked to a number of the people who question the official version.
Dylan Avery, the 23-year-old film-maker behind the internet film Loose Change says the US government “will willingly kill its own citizens for whatever gain it seems necessary and then lie as much as they need to cover it up.”
Alex Jones, a Texan nationally syndicated radio talk show host, tells the programme “9/11 is an inside job… a false-flag terror operation.”
Jim Fetzer, former US marine and retired university professor, who helped found a coalition of academics called Scholars for 9/11 Truth repeats the Sherlock Holmes quotation “when you have eliminated the impossible, whatever remains, however improbable, must be the truth.”
There has been an official fightback of sorts.
President Bush is on record imploring people to reject conspiracy theories: “Let us never tolerate outrageous conspiracy theories concerning the attacks of September the 11th” which he said were “malicious lies that attempt to shift the blame away from the terrorists, themselves, away from the guilty.”
The US State Department has a website to debunk conspiracy theories – not just about 9/11 but a whole range of stories circulating on the internet.
But we found that simple requests, such as asking to see the plane wreckage of flight United 93 at Shanksville, or flight American Airlines 77 at the Pentagon, were refused after months of delay by the authorities.
Yet if we had been able to film the wreckage from flight AA77 we would have had extremely strong evidence that a Boeing 757 hit the Pentagon.
Trying to prove or disprove these alternative theories is not easy.
Officials are loathe to engage, thinking that any response will only fan the flames of popular conspiracy theories, and yet no response seems to be worse still.
9/11: The Conspiracy Files travels across the United States investigating the allegations and talking to witnesses wherever possible.
Ultimately you can’t beat speaking to eyewitnesses, such as the local coroner at Shanksville, Wally Miller.
Wally Miller tells the programme how comments he made about the wreckage at Shanksville have been misquoted on the internet by people who do not “take the trouble to come here and ask me about it.”
Miller is quoted as saying, “I stopped being coroner after about 20 minutes because there were no bodies there”; yet he also said it was perfectly clear that the manner of death was a plane crash, and the point he was trying to make was that it had become a large funeral service.
The Conspiracy Files also talked to Senator Bob Graham who co-chaired the Congressional Inquiry into 9/11 which detailed the failure of the CIA and FBI to use intelligence it had received about Al Qaeda before the attacks.
Senator Graham told us there was a “collaboration of efforts among agencies and the administration to keep information out of the public’s hands.”
“Within 9/11 there are too many secrets” adding that “withholding of those secrets has eroded public confidence’ in security”.
And crucially we may not have learned about that conspiracy without the questioning of every aspect of the official version.