That's not exactly what Clarke said. I suggest reading his book. Most of the finger pointing in it goes to the FBI, actually. While all the news folks hype the Clarke/Rice debate, he really says almost nothing bad about her in the book, other than the whole not being interested in Al-Quaeda thing. And I don't mean that to sound ironic, because in the context of everything else he wrote, that was really a very minor issue. He's much harsher on Ashcroft, who's priority list didn't even mention terrorism, but was focused on things like pornography and drugs.
Fact is, two of the hijackers were known terrorists, who were not supposed to get on planes or even be in America before 9/11. But the FBI never shared their information with the FAA. If they had caught the two of them, at the very least, there would have been one less plane hijacked. Combine that with the reports from Phoenix and Minnesota FBI, and it's pretty clear where the screwups happened.
Further, look at the way Ashcroft handled the Jose Padilla case. If you want to find out what terrorists are doing in America, it seems like the best way to go about gather intelligence would be to follow them and see who they were meeting with. When they arrested him, they knew he had no workable plans, and that he wasn't an immediate threat. But instead of doing the long slog of surveillance work, they just grabbed him at the airport so Ashcroft could have a nice press conference. But do they know anything about his associates? Whether there was a sleeper cell in place in America with which he was aligned? They don't know any of that, because they took the flash of an arrest over the substance of the detective work. This government only cares about looking good, regardless of the consequences. They govern by keeping the populace in a constant state of fear. It's abhorrent that this is happening in what is supposed to be a free country.