People walking amid the debris near the World Trade Center in 2001
Accusations that the American Government was complicit in the 9/11 attacks are becoming increasingly bitter and widespread.
9/11 was the first global event in the age of the internet. And now the world wide web is being used as a platform for a wide range of conspiracy theories - more than 50 at the last count - which allege that the US government was somehow involved in the attacks on the World Trade Centre and the Pentagon.
In the past, you needed the backing of a Hollywood studio or a major broadcaster to reach a global audience. Now, all you need is a bargain basement computer and a little technical know-how.
Dylan Avery is a case in point. He's the 23-year-old documentary maker behind an internet phenomenon.
When he was turned down by film school, he decided to make his own movie - and distribute it over the world wide web.
Loose Change has been viewed tens of millions of times and pulls no punches when it comes to saying who he believes is behind 9/11.
"Our government will willingly kill its own citizens for whatever gain it deems necessary," he told The Conspiracy Files, "and then lie as much as they need to cover it up."
An accusation the US Government categorically denies.
Those who question the official account of 9/11 form a broad coalition. At the other extreme to Dylan Avery is Professor Jim Fetzer.
Fetzer is a former US Marine officer and retired professor of philosophy. Puzzled by the apparent discrepancies in the official account of 9/11, he founded a coalition of like-minded academics called Scholars For 9/11 Truth.
Its purpose is to research exactly what happened that day, using the principles of scientific research.
"Sometimes truth is stranger than fiction," says Professor Fetzer. "As Sherlock Holmes was fond of observing, 'when you have eliminated the impossible whatever remains, however improbable, must be the truth.'"
The members of Scholars for 9/11 Truth refuse to accept that the attacks were just down to Al Qaeda.
"The very idea," says Fetzer, "that 19 Islamic fundamentalists could have hijacked these four commercial airliners, outfoxed the most sophisticated air defence system in the world, under control of a man in a cave in Afghanistan is only the most outrageous of the conspiracy theories."
"That has forced us in the direction of a deeper and darker complicity by officials of our own government."
Conspiracy theorists are tapping in to a rich vein of cynicism across America.
Alex Jones, a nationally syndicated radio talk show host, is a leading voice in the self-styled 9/11 Truth Movement. He believes 9/11 was "an inside job."
"It's a self inflicted wound, it's a false-flag terror operation," he claims. "We're going to expose the lies one by one, we're never going to stop and we will prevail, we will win, we will never surrender."
But the US Government is fighting back. President Bush has said: "We must speak the truth about terror. Let us never tolerate outrageous conspiracy theories concerning the attacks of Sept 11th."
And to try and fight the internet bloggers on their own territory, the US State Department has launched a website to debunk conspiracy theories - not just about 9/11 but a whole range of urban myths and inaccurate stories circulating on the internet.
After the attacks, government officials were summoned to give evidence before a Congressional Inquiry set up to investigate the intelligence failure before 9/11.
Co-chairman Senator Bob Graham told The Conspiracy Files of his frustration at the lack of co-operation from the FBI in that inquiry, and by the government's decision to censor over 30 pages of his report which related to Saudi Arabia.
"Within 9/11 there are too many secrets," he said, "and that withholding of those secrets has eroded public confidence in their government as it relates to their own security."
The Conspiracy Files investigated the many questions that have been raised to find out what really happened on 9/11. You can read the results of that research by clicking on the links below.
The Conspiracy Files: 9/11 was broadcast on Sunday, 18 February 2007 at 2100 GMT on BBC Two.