There are 5 levels of possible involvement of the Bush Administration. 1. None, they were completely surprised. 2. They knew it was going to happen but couldn't stop it. 3 They knew and let it happen. 4 They helped it happen. 5. They made it happen.
Anyone in categories 3, 4, or 5, are considered "truthers". Anyone not in those categories are considered believers in either the "incompetence theory" or "coincidence theory" or some combination of the two. The incompetence theory implies that the FAA and military people involved were so incredibly incompetent as to let this disaster happen. You have to admit that there must have been a lot of screw ups going on that day. Just a quick google will reveal how many times the US fighter jets were sent in the wrong direction on orders from unknown sources. Did you know that military leaders have testified twice and told two different stories of the events of that day? And these stories came 3 years apart. The stories were so different that the Commission threatened to indict the generals. The final stories implies that the FAA was so incredibly incompetent that no military plane was even in the air before the hijacked planes they were launched to chase, had already crashed. Keep in mind, the Pentagon was hit more than 90 minutes after the WTC. Man, that is incompetent. No one was demoted, or fired, or even scolded. (Note that by telling two different stories three years apart, the military has admitted they were involved in a cover-up, since if the first story was wrong, people knew about it for three years and covered it up. If the second story is the lie, well then that story is the cover up.)
The coincidence theorist believe that everything just went right for the terrorists that day, and nothing went right for us. Any idea how hard it is to hit a building with a jet doing 500 mph at near sea level? It's nearly impossible. But that is the approximate impact speeds of all three planes. Trained pilots in simulators managed the task 1 in 100 times. Man, those terrorists were lucky that day. Three hits in a row by inexperienced pilots carries odds of 1:1,000,000 or more (The Pentagon hit was even harder to do. They hit the first floor of a four story building without ever touching the lawn. Amazing at 500 mph.). They should have bought a lottery ticket that day.
So if the incompetency theory or the coincidence theory seem to stretch beliefs, it leave only the truther theories. Remember what Sherlock Homes said, "Once you eliminate the impossible, whatever remains, no matter how improbable, must be the truth."
That is why I am a truther, and proud of it.