From the show Conspiracy Theory on the Pentagon strike on 9/11:
Perhaps as significant as what is unknown about the Pentagon attack is what has been forgotten. Just prior to the September 11 attacks in 2001, it was announced that the Department of Defense could not account for more than $2.3 trillion dollars. Donald Rumsfeld was questioned about it the day before by Rep. Cynthia McKinney, with little to say in accounting for the astonishing sum of money. But all the evidence of this accounting fraud, or error, was destroyed in the 9/11 attack during which the Pentagon was fortuitously struck in the very area where some of this evidence was being held for investigation. Oddly, a great deal of important financial & investigative information was also destroyed in the collapse of WTC7 on the same day.
Here is the segment from the show: