It is an aeroplane.
Let me explain why.
If it isn't a plane, 9/11 is a government conspiracy, and in that case it still has to be plane.
Here is a fact. Same day, 9/11, they announce officially, that a plane hit Pentagon.
It turns out, that there isn't any clear pictures about the plane. This is pointed by some to be a 'proof' about said conspiracy.
But, and this is important, US government couldn't have known that there isn't any pictures beforehand!
Think about these 'super-conspirators'. They think everything, right? Including the fact, that they can't before the event say, that there isn't some tourist, nature-photographer, would-be architect or just somebody taking picture of his girlfriend and their missile or whatever could have ended pictured clearly in that picture, shown on evening news and collapsing entire US-government, propably getting a lynch mob on White House lawn.
So, if there isn't a conspiracy, it is a plane.
And if there is a conspiracy, it still has to be a plane.
So, the thing in the picture is a plane. QED