Concern has been growing over the solvency of some banks amid the housing slump and the steep slide in the mortgage market. The pressures of tighter credit, tumbling home prices and rising foreclosures have been battering banks of all sizes nationwide.

The FDIC has been keeping an especially close eye on banks and thrifts with high levels of exposure to the riskiest borrowers and markets, agency officials say, including subprime mortgages and construction loans in overbuilt areas.

Another area of potential concern: banks' holdings of preferred stock of troubled mortgage giants Fannie Mae and Freddie Mac. A government rescue of the companies, whose share prices have rebounded a bit this week after plummeting recently as they struggle with billions of dollars in losses from bad mortgages, could be costly for scores of banks that hold billions in their preferred shares.

"We're closely monitoring that situation," Bair said.

The FDIC said troubled assets — loans that are 90 or more days past due — continued to rise in the second quarter, jumping by $26.7 billion, or 19.6 percent, over the first quarter. It was the first time since 1993 that the percentage of total loans that were troubled broke 2 percent, at 2.04 percent.

The agency doesn't disclose the names of institutions on its internal list of troubled banks. On average, 13 percent of banks that make the list fail.

Pasadena, Calif.-based IndyMac was taken over by the FDIC on July 11 with about $32 billion in assets and deposits of $19 billion. It was the second-largest financial institution to close in U.S. history, after Continental Illinois National Bank in 1984.