Follow the money.
Agreed. The 9/11 Commission, on the other hand, argued that the question was of "little practical significance":
**
To date, the U.S. government has not been able to determine the origin of the money used for the 9/11 attacks. Ultimately the question is of little practical significance.
**
Source:
National Commission on Terrorist Attacks Upon the United States, Chapter 5: Al Qaeda aims at the American Homeland