But during the credit market upheaval in August, 14 percent of trades in these contracts were unconfirmed, meaning one of the parties in the resale transaction was unidentified in trade documents and remained unknown 30 days later. In December, that number stood at 13 percent. Because these trades are unregulated, there is no requirement that all parties to a contract be told when it is sold.

As investors who have purchased such swaps try to cash them in, they may have trouble tracking down who is supposed to pay their claims.

“This is just a giant insurance industry that is underregulated and not very well reserved for and does not have very good standards as a result,” said Michael A. J. Farrell, chief executive of Annaly Capital Management in New York. “I think unregulated markets that overshadow, in terms of size, the regulated ones are a real question mark.”

Because these contracts are sold and resold among financial institutions, an original buyer may not know that a new, potentially weaker entity has taken over the obligation to pay a claim.

In late 2005, at the urging of the Federal Reserve Bank of New York, market participants agreed to advise their trading partners in a swap when they assigned contracts to others. But it is unclear how closely participants adhere to this practice.

It would be as if homeowners, facing losses after a hurricane, could not identify the insurance companies to pay on their claims. Or, if they could, they discovered that their insurer had transferred the policy to another company that could not cover the claim.

Credit default swaps were invented by major banks in the mid-1990s as a way to offset risk in their lending or bond portfolios. At the outset, each contract was different, volume in the market was small and participants knew whom they were dealing with.