It is popular, particular with right-wingers, for example, to blame the whole financial crisis on Fannie Mae and Freddy Mac, but clearly this does not account for the fact that the ratings institutions themselves were giving Fannie and Freddie top ratings up until the day they were taken over. Furthermore, a scenario like this wouldn't explain the collapse of Countrywide, or AIG where the government oversite and influence was effectively non-existent.
No, the rabbit hole goes far deeper. When Congress passes laws that specifically forbid the regulation of derivatives, and repealed Glass-Steagall simply so that Citibank could merge with Traveller's, one has to say that there is more afoot than a few bad loan practices. The industry was systematically restructured so that would not have the accountability or regulation that traditionally protected the system since the Great Depression.
When one combines this with the fact that many of our treasury officials, like Summers and Paulson have been in leadership positions at Merrill Lynch, one cannot help but question the incestuous nature of our financial sector.