I'm with Nickbat and Lizzie on this one 
Me too, I stopped believing in fairy tales many years ago.
Many conspiracy stories appear to be very feasible because their originators WANT you to believe it. 
They're called conspiracy THEORIES because that's exactly what they are......theories.
But, the only reason anyone believes it was a terrorist attack is because they have been TOLD it was a terrorist attack.
We have all been TOLD it was this guy or that guy and all at the hands of Bin Laden.
I find it funny how people are so ready to believe when they are TOLD a handfull of guys from the Middle East can cause so much destruction, yet find the idea of it being a false flag operation instigated by the biggest world power completely implausible.
Bin Laden has not accepted resposability, it isn't even listed on his FBI rap sheet, yet it must be him and Al-Qaeda. I would rather form my opinions on fact and evidence, than what I'm told to believe.
As I said earlier, Operation Northwoods shows that these false flag operations are a possibility.
Until proved either way, I will keep an open mind.