Evidence outlined in a Pentagon contractor report suggests that financial subversion carried out by unknown parties, such as terrorists or hostile nations, contributed to the 2008 economic crash by covertly using vulnerabilities in the U.S. financial system.

Suspects include financial enemies in Middle Eastern states, Islamic terrorists, hostile members of the Chinese military, or government and organized crime groups in Russia, Venezuela or Iran.
We think it is most likely the US did it to herself. So that attack can be justified. This is a common tactic (please see the last 300 years of military documentation coming out of any govenrment).

Financial terrorism suspected in 2008 economic crash - Washington Times