I wrote in May 2009 on how hedge funds could squeeze a bank:

There were two dimensions to the cause of the current credit crisis. The first was that unit risk was not eliminated, merely transferred to a larger pool to make it invisible statistically. The second, and more ominous, was that regulatory risks were defined by credit ratings, and the two fed on each other inversely. As credit rating rose, risk exposure fell to create an under-pricing of risk. But as risk exposure rose, credit rating fell to exacerbate further rise of risk exposure in a chain reaction that detonated a debt explosion of atomic dimension.

The Office of the Comptroller of the Currency and the Federal Reserve jointly allowed banks with credit default swaps (CDS) insurance to keep super-senior risk assets on their books without adding capital because the risk was insured. Normally, if the banks held the super-senior risk on their books, they would need to post capital at 8% of the liability. But capital could be reduced to one-fifth the normal amount (20% of 8%, meaning $160 for every $10,000 of risk on the books) if banks could prove to the regulators that the risk of default on the super-senior portion of the deals was truly negligible, and if the securities being issued via a collateral debt obligation (CDO) structure carried a Triple-A credit rating from a “nationally recognized credit rating agency”, such as Standard and Poor’s rating on AIG.

Read more here.