State Street Corp., BlackRock Inc., Societe Generale and Deutsche Bank AG units are among 28 managers of collateralized debt obligations deemed at risk of being unable to pay off the most-senior classes after a slump in the credit quality of their mortgage holdings. About $64 billion of CDOs, which repackage pools of assets into new securities with varying risks, have experienced so-called events of default since mid-October, according to data sent to clients yesterday by Charlotte, North Carolina-based Wachovia Corp. analyst Justin Pauley. CDOs containing mortgage bonds represented the largest source of about $100 billion of debt-related losses at the world's largest financial firms last year as record subprime loan delinquencies, surging defaults on other home loans and the highest foreclosure rates on record led to unprecedented downgrades and price drops among mortgage bonds and related CDOs.