Mr Clarke, who now has a consultancy firm in Arlington, Virginia, remains uncertain whether al-Qa'ida could have been stopped. "I don't think we know. It's very facile to say it could have been or could not have been. There is absolutely no way of knowing. What I do believe is that had we known about the two al-Qa'ida individuals who were among the hijackers ... Had we known they were in the country, which the FBI at some level knew and which the CIA at some level knew, had my counterparts at the FBI and CIA known, had I known, then I firmly believe we could have caught those two.
http://www.informationclearinghouse.info/article6320.htm

"Now, you can draw all sorts of conclusions from that. One, is that, simply, there would have been 17 hijackers. Another conclusion is that we might have been able to pull strings on those two and find more of the 19. But even if we had rounded up all 19 there would have been another 19. There would have been another major attack. The point is that al-Qa'ida was on a march to have a major terrorist attack ... They would not stop until they succeeded in having one. So yes, we might have been able to stop a particular attack."

Apart from the missed opportunities he highlights, what might be of potentially greater concern is Mr Clarke's belief that al-Qa'ida could easily attack again, and America and Britain remain exceedingly vulnerable. Another attack is not inevitable ("I think almost nothing is inevitable," he said) but possible.

He added: "I think it is harder but I can think of ways of them doing it and I'm sure they can imagine ways of doing it. It's entirely possible there will be another major attack." A dirty bomb, he believes, is probably in the "too hard" category. It is more likely terrorists would use suicide-bombs to attack softer targets, such as casinos or shopping malls. "Those are the two scenarios I use all the time when discussing it," he said. "If you do eight guys in eight shopping malls you have an enormous effect on the economy ... so much of the US economy is tied up with retail sales.

"If you did four casinos with four guys you could destroy the economy of Las Vegas. There are lots of low-end ways of doing things. And the reason they have not done some of the low-end threats, I think, is because they set the barrier for themselves very high with the 9-11 attacks. They may want another major attack; they may feel that if they do less than a major attack [they] will look like a lesser force."

"[Iraq] is a fiasco," he said. "We can only hope there is a way of minimising the losses and getting out in a way that allows us to leave behind some sort of stable government. If [it stays as it is] now there is a high risk that what we leave behind will be worse than what was there before ... Iraq could easily be much more of a problem for us than it would have been if Saddam Hussein had stayed in power."