This is not the conclusion of a scientific survey, but it is what routinely comes up in conversations around the region — in a shopping mall in Dubai, in a park in Algiers, in a cafe in Riyadh and all over Cairo.

“Look, I don’t believe what your governments and press say. It just can’t be true,” said Ahmed Issab, 26, a Syrian engineer who lives and works in the UAE. “Why would they tell the truth? I think the US organised this so that they had an excuse to invade Iraq for the oil.”

It is easy for Americans to dismiss such thinking as bizarre. But that would miss a point that people in this part of the world think Western leaders, especially in Washington, need to understand: That such ideas persist represents the first failure in the fight against terrorism — the inability to convince people here that the US is, indeed, waging a campaign against terrorism, not a crusade against Muslims.

“The US should be concerned because in order to tell people that there is a real evil, they too have to believe it in order to help you,” said Mushairy al-Thaidy, a columnist in the Saudi-owned regional newspaper Asharq al Awsat. “Otherwise, it will diminish your ability to fight terrorism. It is not the kind of battle you can fight on your own, it is a collective battle.”

There are many reasons people here said they believed that the attacks of 9/11 were part of a conspiracy against Muslims. Some had nothing to do with Western actions, and some had everything to do with Western policies.

Again and again, people said they simply did not believe that a group of Arabs — like themselves — could possibly have waged such a successful operation against a superpower like the US. But they also said that Washington’s post-9/11 foreign policy proved that the US and Israel were behind the attacks, especially with the invasion of Iraq.

The rumours that spread shortly after 9/11 have been passed on so often that people no longer know where or when they first heard them.