Would the real 20th hijacker please stand up...

Supposedly, a man named Mohamed al Kahtani who is being held at Gitmo was supposed to be the "20th hijacker" on 9/11.

Up until recently, we had been told that a man named Zacarias Moussaoui was the "20th hijacker".

Why does it seem like whenever the Bush administration gets backed into a corner, or called upon to explain itself, it comes up with weak rhetoric about liberty or some vague reference to the 9/11 attacks?

Does the government actually know who it has at Gitmo and what value these people may provide?