http://www.adl.org/learn/jttf/wtcb_jttf.asp
This article discusses the first WTC bombing in which numerous people were caught and convicted.
In reading an article on Tenet's resignation, I found this:
The CIA is expected to be harshly criticized in the final report due July 26 from the commission investigating the Sept. 11 attacks. In a preliminary report, the panel found the CIA didn't even identify the al-Qaeda network until 1999 - 11 years after it was formed. Tenet did, however, issue repeated warnings about the threat posed by Osama bin Laden.
So, if the presence of the al-Qaeda network wasn't known until 1999, nobody could have done much about it. We can also be pretty sure that when/if we capture bin Laden, it won't be the end of al Qaeda. In fact, it will probably trigger more terror incidents. Not that we shouldn't go after bin Laden anyway, but they have said that the 9/11 incident was in the planning since 1996, so it's pretty tough to say that anybody could have prevented it.
Al Qaeda is responsible for 9/11, not Bush, not Clinton, not Bush Sr., not anybody, but Al Qaeda. If they were determined to do a similar attack today, I doubt anybody would stop them, even now.
Dee