Shares in the world's largest insurer, American International Group (AIG), have fallen by more than 11% after its auditors voiced concerns.
There was a "material weakness" in how AIG valued defaults on credit, the auditors PricewaterhouseCoopers said.
AIG, the shirt sponsor of football team Manchester United, has previously said that it did not face major exposure to problems in the US housing market.
However, analysts said the auditors' comments put a question mark over this.
AIG has divisions which insure and invest in sub-prime loans, which are made to people with poor or non-existent credit histories.
In recent months, sub-prime loans have seen a surge in defaults, a factor that contributed to the recent and sudden freezing of world credit markets.
AIG said it was "still accumulating market data in order to update its valuation" of the sub-prime portfolio.
However traders feared that the firm might have more mortgage debt to write off.
By the close of trading in New York, AIG had dropped $5.94, or 11.7% to $44.74.