Where was the US air force on the morning of 11 September 2001? Why were fighter planes not scrambled immediately it was suspected that a hijacking was taking place? Why, despite repeated intelligence warnings of a terrorist hijack, did the US not act before 11 September against any of the attackers? Why did five of them apparently undergo military training in the US during the 1990s? And why, since 9/11, have US forces failed to capture Osama Bin Laden?
Add all these questions together and you get a much bigger one: did the Bush administration know the 9/11 attacks were coming, and did they let them happen? These questions hit e-mail lists and websites within hours of the attacks, and have not gone away. Michael Meacher, Tony Blair's environment minister from 1997 to 2003, raised them in an article in the Guardian on 6 September, and caused a firestorm of both criticism and support.
Meacher insists that he was simply "asking important questions, and asking for an investigation. I am not saying the US government allowed it to happen. At no point does my judgement intrude on that question."
The article was a critique of US foreign policy, with which, he says, "the UK government has been far too complicit". The Bush administration plans, he wrote, had been drawn up in the late 1990s: the need to take military control of the Gulf; the need for regime change in Iraq, Iran and even China; the imperative of securing oil supplies from the Gulf to the Caspian; and the need to "discourage advanced industrial nations from challenging our leadership or even aspiring to a larger regional or global role". All this made up "a blueprint for US world domination", he wrote.
That was already enough to infuriate the establishment, particularly in the US, which sent its ambassador in London on to the airwaves to dismiss Meacher as a nonentity. But it was the questions about 9/11 that really got the pundits going. Meacher cannot contain his irritation. "It's a classic Aunt Sally operation," he fumes. "In order to reject a thesis which is extremely uncomfortable for them, they hit back at me and resort to smears."
What worries some observers, more than the pretext for Bush's actions, is that Meacher's article could provide a pretext for a new round of attacks on the green movement, for which he has become something of a media representative.
Since leaving office, Meacher has been taking up green causes with relish - calling for more dramatic action on climate change, criticising the government's love affair with GM crops and warning about the planet's future. But could his questions about 9/11 now be used to support the views of those who say the green movement's arguments are based on exaggeration, conspiracy and wishful thinking?
Green-bashers come from across the political spectrum. From the right - from business, politicians and extremist think-tanks that deny the existence of everything from climate change to overfishing. From supposed ex-greens who say they have now seen the light - from the Greenpeace co-founder Patrick Moore to the media darling Bj0rn Lomborg.
Tony Juniper, director of Friends of the Earth, chooses his words carefully: "I think Michael would speak for many people in voicing doubts about the Bush administration," he says. "But the evidence of their intent is there in everything from the destruction of the Kyoto Protocol to the hijacking of the Johannesburg Earth Summit. I would simply say there is no need to resort to theory when you have so many facts on the table."
Zac Goldsmith, editor of the Ecologist, agrees: "Meacher would have been wiser to focus on what we know to be true - that the Bush administration has mercilessly exploited the tragedy to advance an authoritarian and corporatist agenda."
Meacher is having none of it. He is not a "spokesman" for anyone, he says. If the usual suspects choose to move on from smearing him to smearing greens in general, it amounts to "an argument of such mind-blowing stupidity that it is hardly worth commenting on".
"What is so appalling," he insists, "is that this whole argument gets reduced to minutiae - who inserted the 45-minute claim; is this a conspiracy? - rather than being used to look seriously at the real reasons for US foreign policy. Not once, as far as I have seen, has anyone taken up my questions in the media. Instead, they have just utterly denigrated them. I think this says far more about them than about the questions."
Paul Kingsnorth's One No, Many Yeses is published by the Free Press (£10)