Six years after 9/11, the American public have still not been provided with a full and truthful account of the single greatest terror attack in US history.

What they got was a turkey. The 9/11 Commission was hamstrung by official obstruction. It never managed to ascertain the whole truth of what happened on September 11 2001.

The chair and vice chair of the 9/11 Commission, respectively Thomas Kean and Lee Hamilton, assert in their book, Without Precedent, that they were "set up to fail" and were starved of funds to do a proper investigation. They also confirm that they were denied access to the truth and misled by senior officials in the Pentagon and the federal aviation authority; and that this obstruction and deception led them to contemplate slapping officials with criminal charges.

Despite the many public statements by 9/11 commissioners and staff members acknowledging they were repeatedly lied to, not a single person has ever been charged, tried, or even reprimanded, for lying to the 9/11 Commission.

From the outset, the commission seemed to be hobbled. It did not start work until over a year after the attacks. Even then, its terms of reference were suspiciously narrow, its powers of investigation curiously limited and its time-frame for producing a report unhelpfully short - barely a year to sift through millions of pages of evidence and to interview hundreds of key witnesses.

The final report did not examine key evidence, and neglected serious anomalies in the various accounts of what happened. The commissioners admit their report was incomplete and flawed, and that many questions about the terror attacks remain unanswered. Nevertheless, the 9/11 Commission was swiftly closed down on August 21 2004.

I do not believe in conspiracy theories. I prefer rigorous, evidence-based analysis that sifts through the known facts and utilises expert opinion to draw conclusions that stand up to critical scrutiny. In other words, I believe in everything the 9/11 Commission was not.

The failings of the official investigation have fuelled too many half-baked conspiracy theories. Some of the 9/11 "truth" groups promote speculative hypotheses, ignore innocent explanations, cite non-expert sources and jump to conclusions that are not proven by the known facts. They convert mere coincidence and circumstantial evidence into cast-iron proof. This is no way to debunk the obfuscations and evasions of the 9/11 report.

But even amid the hype, some of these 9/11 groups raise valid and important questions that were never even considered, let alone answered, by the official investigation. The American public has not been told the complete truth about the events of that fateful autumn morning six years ago.

What happened on 9/11 is fundamentally important in its own right. But equally important is the way the 9/11 cover-up signifies an absence of democratic, transparent and accountable government. Establishing the truth is, in part, about restoring honesty, trust and confidence in American politics.

There are dozens of 9/11 "truth" websites and campaign groups. I cannot vouch for the veracity or credibility of any of them. But what I can say is that as well as making plenty of seemingly outrageous claims; a few of them raise legitimate questions that demand answers.

Four of these well known "tell the truth" 9/11 websites are:

1) Scholars for 9/11 Truth, which includes academics and intellectuals from many disciplines.

2) 250+ 9/11 'Smoking Guns' a website that cites over 250 pieces of evidence that allegedly contradict, or were omitted from, the 9/11 Commission report.

3) The 911 Truth Campaign that, as well as offering its own evidence and theories, includes links to more than 20 similar websites.

4) Patriots Question 9/11, perhaps the most plausible array of distinguished US citizens who question the official account of 9/11, including General Wesley Clark, former Nato commander in Europe, and seven members and staffers of the official 9/11 Commission, including the chair and vice chair. In all, this website documents the doubts of 110+ senior military, intelligence service, law enforcement and government officials; 200+ engineers and architects; 50+ pilots and aviation professionals; 150+ professors; 90+ entertainment and media people; and 190+ 9/11 survivors and family members. Although this is an impressive roll call, it doesn't necessarily mean that these expert professionals are right. Nevertheless, their scepticism of the official version of events is reason to pause and reflect.

More and more US citizens are critical of the official account. The respected Zogby polling organisation last week found that 51% of Americans want Congress to probe President Bush and Vice-President Cheney regarding the truth about the 9/11 attacks; 67% are also critical of the 9/11 Commission for not investigating the bizarre, unexplained collapse of the 47-storey World Trade Centre building 7 (WTC7). This building was not hit by any planes. Unlike WTC3, which was badly damaged by falling debris from the Twin Towers but which remained standing, WTC7 suffered minor damage but suddenly collapsed in a neat pile, as happens in a controlled demolition.

In a 2006 interview with anchorman Evan Soloman of CBC's Sunday programme, the vice chair of the 9/11 Commission, Lee Hamilton, was reminded that the commission report failed to even mention the collapse of WTC7 or the suspicious hurried removal of the building debris from the site - before there could be a proper forensic investigation of what was a crime scene. Hamilton could only offer the lame excuse that the commissioners did not have "unlimited time" and could not be expected to answer "every question" the public asks.

There are many, many more strange unexplained facts concerning the events of 9/11. You don't have to be a conspiracy theorist to be puzzled and want an explanation, or to be sceptical concerning the official version of events.

Six years on from those terrible events, the survivors, and the friends and families of those who died, deserve to know the truth. Is honesty and transparency concerning 9/11 too much to ask of the president and Congress?

What is needed is a new and truly independent commission of inquiry to sort coincidence and conjecture from fact, and to provide answers to the unsolved anomalies in the evidence available concerning the attacks on the World Trade Centre and the Pentagon. Unlike the often-stymied first investigation, this new commission should be granted wide-ranging subpoena powers and unfettered access to government files and officials. George Bush should be called to testify, without his minders at hand to brief and prompt him. America - and the world - has a right to know the truth.