The 9/11 conspiracy theory is back, in a much more virulent form, and normally sane people are being taken in by it. I get half a dozen earnest emails every day telling me I must see a film called Loose Change.
It has been around in various versions for almost two years, but it now seems to be gathering converts faster than ever.
Well, I have seen it, and I concede that it is a much slicker, more professional product than other 9/11 conspiracy films, and therefore more seductive.
But the argument is pure paranoid fantasy and it is rotting people's brains.
There have always been two versions of the 9/11 conspiracy theory. The lesser version held that the Bush Administration had advance intelligence of al Qaeda's plans but chose to ignore the warning because the attacks suited its purposes.
The greater version insisted that there was no al Qaeda involvement, and that the attacks were carried out by the United States Government.
Until recently, the greater version was largely confined to the Arab world, where many people are in complete denial about any Arab involvement in the atrocity.
Very few Americans took that version seriously, although many wondered whether the intelligence lapses had really been accidental.
Even the lesser conspiracy would have required the complicity of half a dozen very senior people who received the intelligence and decided to ignore it: the heads of the CIA and the FBI (George Tenet and Louis Freeh), the national security adviser (Condoleezza Rice), the Secretaries of Defence and State (Don Rumsfeld and Colin Powell), plus Vice-President Cheney and perhaps President Bush.
It would also have required the permanent silence - or silencing - of a dozen lower-level intelligence analysts who knew that the senior people had seen the information.
I don't believe that happened because I don't think that Tenet, Rice, Powell et al would have deliberately plotted the deaths of thousands of Americans.
I don't believe even Dick Cheney would have done that. And I note that there has been no inexplicable wave of sudden deaths among junior intelligence analysts in Washington.
I do believe, however, that 9/11 served the purposes of the neo-conservatives and their allies in the Bush administration.
They were already planning to attack Iraq, as part of a larger plan, dating back to the late 1990s and the Project for a New American Century, to re-launch Pax America and re-establish American hegemony in the 21st-century world. I agree that they were adroit in seizing on 9/11 as a way of enlisting popular support for their project. But that's all.
I cannot absolutely refute the lesser theory, but find it implausible. The greater conspiracy theory, on the other hand, is just plain loony - and yet more and more people are falling for it in the West, where it was once the exclusive domain of people with counter-rotating eyeballs and poor personal hygiene. You cannot overstate the impact of a well-made film.
Loose Change confidently asserts that the twin towers were brought down by carefully placed demolition charges, not by the fires ignited by the planes that hit them; that the Pentagon was struck by a cruise missile, not by a plane; and that the fourth hijacked plane, Flight 93, did not crash in a field in Pennsylvania but landed at Cleveland where the passengers were taken into a Nasa building and never seen again.
What about all the calls that the passengers on Flight 93 made on their phones?
Their voices were cloned by the Los Alamos laboratories and the calls to their relatives were faked.
The FBI was in on it, the CIA was in on it, the Air Force was in on it - except those who were killed at the Pentagon - and North American Aerospace Defence Command was in on it.
The security companies guarding the World Trade Centre were in on it, Mayor Rudy Giuliani was in on it, the Federal Aviation Administration was in on it, Nasa was in on it, and the Pentagon was in on it. At least 10,000 people were in on it. They had to be, or it couldn't have worked.
More than five years later, not one of them has talked. Nobody has got drunk and spilled their guts. Nobody has told their spouse, who then blabbed.
Not one of these 10,000 accomplices to mass murder has yielded to the temptation for instant fame and great wealth for blowing the whistle on the greatest conspiracy in history.
In normal times you wouldn't waste breath arguing with people who fall for this kind of rubbish. But the makers of Loose Change claim their film has already been seen by more than 100 million people, and looking at my email in-tray I believe them.
It is a real problem, because by linking their fantasies about 9/11 to the Bush Administration's deliberate deception of the American people in order to gain support for the invasion of Iraq, they bring discredit on the truth and the nonsense alike.
You almost wonder if they are secretly working for the Bush Administration.
* Gwynne Dyer is a London-based independent journalist whose articles are published in 45 countries.