Wall Street banks are in for about $64 billion in total write-downs related to collateralized debt obligations, as banks have likely not reported the full extent of their losses, according to Citigroup research.

Recent credit market turmoil has resulted in large part from the devaluation of CDOs, or bundles of subprime loans repackaged into securities. As borrowers with poor credit have been increasingly making late payments and defaulting on loans, CDOs attractiveness for investors has fallen, leaving banks carrying the risk.

Of the many skeletons hiding in the subprime closet, write-downs on banks' positions on CDOs of (asset-backed securities) are probably the scariest, stated analysts led by Matt King in a report Wednesday.

The analysts, which did not include potential Citigroup write-downs, noted that banks may have not completely reported their full array of devalued securities. In its latest quarterly report, brokerage Merrill Lynch reported a $7.9 billion write-down in mortgage related securities. The analysts estimated that only about $15 billion in write-downs had so far been exposed.

Was Merrill massively more exposed than everyone else, or have other banks yet to face up to reality? the report stated. In general, we think the latter is true.