Blaming Israel

While a substantial number of those polled believed the US government was in some way behind the attacks, seven per cent point the finger at Israel.

The 2001 attacks prompted the so-called US 'war on terror' [EPA]

"In Muslim countries - where we've carried out a number of focus groups - it's clear there is a feeling that the US had some kind of motivation, such as invading Iraq," Kull said.

"There are also some difficulties with the idea that Muslims carried out attacks on civilians - which is widely seen as wrong and contrary to Islam... And there are plenty of people in Muslim countries who say it would have been too technically difficult [for al-Qaeda] to pull off."

In Indonesia, the world's most populous Muslim nation, a majority 57 per cent said they did not know who was behind the attacks. Only five per cent said that Israel had been involved.

'Conspiracy theorists'

In 2002, the US government set up the 9/11 commission, chaired by Thomas Kean, a former governor of New Jersey, to investigate the event. It published its report in 2004, concluding that the 19 hijackers were all members of al-Qaeda. It also concluded that there had been intelligence failures on the part of the CIA and the FBI, the US's spy agencies.

But the World Trade Centre attacks quickly proved fertile ground for "conspiracy theorists" and sceptics have a wide range of alternative theories to choose from - including that US military personnel were involved in the attack or that the towers were brought down with the help of a controlled explosion.

"There are people who are saying that US soldiers were flying the planes ... or people who say simply that the US government turned a blind eye to the threat or that they somehow got them to do it through some means," said Kull.

One of the proponents of an alternative version of the events has been Willis Carto, editor of the American Free Press newspaper, which frequently points the finger of blame at Israel.

He told Al Jazeera the attacks were "perpetrated by the Israeli Mossad [secret service] in conjunction with the American government".

"There's no real evidence whatsoever that the official story of the planes smashing into the building was true. It's impossible to believe that a few furtive little characters armed with box cutters who had no idea how to fly ... could have manouvred the planes like this," Carto said.

"There are so many holes in the story that no one in his right mind can believe it."

Classroom politics

Those who expound alternative versions of the events have themselves become the targets for debunking. Loose Change, a popular series of internet videos that counters the official version of events, has itself inspired a blog called Screw Loose Change.

More than 2,900 people died in the attacks of September 11, 2001 [GALLO/GETTY]

In 2007, research by Keith Seffen, a senior lecturer in Cambridge University's engineering department, used analysis of an engineering model to show the tower collapse had been "quite ordinary and natural".

The US has also seen the debate enter the university classroom.  Both Ward Churchill, of the University of Colorado, and Kevin Barrett, of the University of Wisconsin, provoked a public outcry over the fact they professed to believe alternative versions of the events.

Their critics feared the academics might be tempted to bring there beliefs into the classroom. Their supporters said that even if they did, a university classroom was the place for alternative theories.

'Structural failure'

Michael Newman, from the National Institute of Standards and Technology (Nist), which worked for several years on reports about how and why the World Trade Centre towers collapsed, told Al Jazeera the debate was unlikely to ever go away.

"Whenever we put out one of our reports, we get phone calls and emails from those with alternative views," he said.

"And you always get a lot of interest around the time of the anniversary - it's a part of history now.

The Nist reports say the towers' collapse was due to "structural failure" after the aeroplanes hit them. The reports have been welcomed by building code regulators and many of Nist's recommendations from its investigations have been adopted, but those with alternative viewpoints have continued to dispute the organisation's findings.

"We've heard opinions from all sides," Newman told Al Jazeera.