The shocking accusation, that the west was involved in plotting, organising, and even carrying out the 9/11 terror atrocities to provide the grounds for the military strikes on the so-called Axis of Evil, remains one of the world's biggest conspiracy theories.

On September 11 2001, the world witnessed the Twin Towers being hit by planes, but some extreme conspiracy theorists claim planted bombs brought them down.

While many of the theories are bizarre, one longstanding view is the towers would not have collapsed in the way they did if they were hit by aircraft.

Some claim there must have been a "controlled detonation" at ground level for the Twin Towers to fall in on themselves as they did.

One key part of their argument is the collapse of a third smaller tower, called Building 7, at the World Trade Centre complex, several hours after the huge skyscrapers fell.

Yesterday, the Northern California 9/11 Truth Alliance held its 12th annual 9/11 Truth Film Festival, where the British documentary Incontrovertible was screened.

The online film by Tony Rooke looks at why Building 7 collapsed in the way that it did and the official explanation given after an investigation by the The National Institute of Standards and Technology, based near Washington DC, who still refuses to release all of its data amid claims it could "compromise public safety".

The 911truth.org is a website which says it is dedicated to exposing a "cover-up" surrounding 9/11, and seeking justice for he victims and their relatives.

A mission statement on its website says: "Our Mission is to expose the official lies and cover-up surrounding the events of September 11th, 2001 in a way that inspires the people to overcome denial and understand the truth.

"Namely, that elements within the US government and covert policy apparatus must have orchestrated or participated in the execution of the attacks for these to have happened in the way that they did."

The website says it also wants justice for all those wronged by the attacks, "beginning with complete disclosure of all records and evidence; reversal of all domestic and foreign policies following from the false premises of the official story”.