Now the Moussaoui trial will again focus on some of these shortcomings. If it is to remain a credible defence, it will attach no importance to notions of LIHOP and MIHOP. There will be many among the relatives of those killed - 1,100 of whom have never been identified or properly buried - looking for further answers, but whether the hearing will dig down and find the truth or just clog the air with the dust it dislodges remains to be seen.

Certainly the issue of what really happened has not gone away for the American public, whose appetite for new theories remains undimmed.

The internet is stuffed with stories of Israeli and Iraqi involvement, profiteering, war triggering and other far-fetched tales which take a number of incredulous assumptions, such as the existence of a nefarious world order and a huge number of people in the loop, as their starting point.

One theory claims Larry Silverstein signed a 99-year lease for the World Trade Centre complex just seven weeks before it was destroyed and subsequently trousered a $3.5bn insurance payment. But it doesn't point out that it will cost Silverstein around $8bn to rebuild the site, in addition to mounting court fees and an annual $120m lease.

Similar theories claim a host of figures, from the president and his brothers on down, stood to make financial capital from the deaths of close to 3,000 people, and pay little regard to the fact that there are easier and far less illegal ways of making money.

The bulk of the theories surround the collapse of the Twin Towers. Most people - and many experts - simply cannot believe that the two structures would collapse not only in the time that they did, but the way they came down. Some believe follow-up bombs were placed at the base of the towers to ensure their collapse.

However, there are some interesting anomalies. Shortly after 9/11, David Schippers, chief prosecutor in the Clinton impeachment, said he was contacted by three FBI agents who mentioned a terrorist attack in September, using a nuclear device in Lower Manhattan. He claims it was common knowledge in the FBI but suppressed by superiors there.

Defence Secretary Donald Rumsfeld was aware of a threat. He revealed to talk show host Larry King that at 8am on September 11, he told a group of Congressmen that "sometime in the next two, four, six, eight, 10, 12 months there would be an event that would occur in the world that would be sufficiently shocking that it would remind people again how important it is to have a strong healthy defence department."

While some government officials said it had been unimaginable prior to the attacks that hijacked planes would be used as weapons, it emerged in the 9/11 Congressional Inquiry that intelligence officers had been told of such an idea at least 12 times in seven years.

The attacks had a huge impact on world economic markets, throwing them into chaos and losing billions of dollars off the price of shares.

But there were a few people getting rich off them. Harvey Pitt, chairman of the Securities and Exchange Commission, whose job it is to police stock and shares dealings, said it had discovered that the number of "put options" purchased on American and United Airlines in the days before 9/11 was six times higher than normal. A put option is an agreement where, for a fee, one party will agree to purchase shares at a fixed price within a certain time frame. It works like a bet on the market.

The New York Stock Exchange also reported that short sales (shares that you borrow to sell) in UAL Corp, whose chief subsidiary is United Airlines, rose 40%, while American Airlines' parent AMR Corp share sales increased 20%, and aircraft manufacturer Boeing Corp rose by 37%.

There was also some remarkably prescient movement on financial organisations. Put options were purchased for Morgan Stanley Dean Witter, which occupied 22 storeys in the World Trade Centre, while Merrill Lynch, which is headquartered nearby, saw a 1,200% increase in put options in the four days before 9/11. It was also reported that $2.5m profits made trading options went unclaimed at re-insurers Munich Re, while investigators from the US Secret Service contacted a number of bond traders over large purchases of five-year Treasury notes - one of the best investments in the event of a world crisis.

While there is no concrete evidence that these activities demonstrated a foreknowledge of events, they add fuel to the fire of the conspiracy theorists. And they appear to be winning the battle.

In a 2004 poll, 49.3% of New York City residents and 41% of citizens from the wider New York area believe the US Government "knew in advance that attacks were planned on or around September 11, 2001, and that they consciously failed to act".

There is also a third theory on 9/11 foreknowledge and its consequences.

Kenneth J Dillon, a historian and former Department of State intelligence analyst, calls it "Anomalous Mistake-driven Opportunity Creation". He says AMOC occurs when an official commits an error so monumentally inept that few will believe he or she did it. Dillon has claimed that Bush and Vice-President Dick Cheney match this criteria in ignoring the "blinking red" system and, if there has been any conspiracy, it has been to hide their incompetence. In light of his administration's woeful response to the destruction wrought by Hurricane Katrina, this doesn't seem too fanciful.

So with the spotlight about to be turned once more on it, is the US government concerned that what happened almost five years ago will be brought up again?

Dr Paul Moorcraft, who formerly worked for the Ministry of Defence and is now the director of the Centre for Foreign Policy Analysis, recently spent time in Washington, meeting with intelligence contacts and other officials. He doesn't think so.

"I didn't have a single conversation about 9/11, and it never cropped up. It's almost as if it's ancient history. Everyone there was far more concerned about how the hell they are going to get out of Iraq, and what they are going to do about Iran," he said.

"There was no talk of the Moussaoui trial.

I'm not sure what can be achieved by raking over 9/11 now. We have new problems to face now. The world has changed a lot in five years."

Lord Anderson agrees.

"It's water under the bridge, we've moved on. There's been a lot of discussion about rendition recently. I saw something in Le Monde, which pointed out that two of the very top al-Qaeda figures (Khalid Sheikh Mohammed, captured in Pakistan in 2003, and Abu Zubaydah, ranked third on the US list of al-Qaeda suspects after Osama Bin Laden and his deputy, Ayman al-Zawahri, and caught in Pakistan in 2002) have disappeared into a black hole.

"As for 9/11, that caravan has moved on."

Prof Rogers thinks the attacks are no longer considered a high priority.

"Nine/Eleven remains significant, because it set off the War on Terror, and it's what the Bush administration harks back to, to justify its actions in Afghanistan and Iraq.

"But it still has big implications for the future, because you cannot maintain control in such a fractured world without tackling the underlying causes.

"I call what we are doing 'liddism'. We are just keeping a lid on what's happening, rather than getting to the problems underneath."

End