How to prove it? Take a look at these images of the fire department response on September 11th, 2001:
(Check out my blog Rare View of Liberty Street Before the South Tower Fell for an additional image.)
I felt the 9/11 images out of New York were doctored since I saw this Peter Turnley image taken from a spot above the Brooks Brothers store in 1 Liberty Plaza on the night of September 11th. Look at the earth-mover resting on a dust-free street. I figured the foregrounds were taken in advance so as to have a supply of stock elements on hand to piece together the imagery.
A primary reason the debris field images were doctored was because there wasn't enough debris left after much of the steel had been dissolved by unknown weaponized means.