Quote from AAAintheBeltway:
...The biggest failures were by the Fed in lax oversight and allowing a real estate bubble to grow unchecked, then acting precipitiously to deflate it, triggering a property crash. You can check the archives and find that I warned at the time that Bernanke was creating a catastrophe.
The other biggest failure was by FNM and FRE,which by that time were more concerned about creating phony profits to justify obscene stock options to the political hacks nominally in charge. The congressmen, like Barney Frank and Chris Dodd, who they had in their back pockets also should have been forced to answer for their role.
Derivatives of course multiplied the scope of the disaster, but they didn't cause it.