He said that, as Countrywide's mortgage portfolio deteriorates, the FHLB exposure "poses an unreasonable risk."
Yet Bigelow said "we are very familiar with FHLB collateral requirements, and we are in full compliance ... The FHLB has some experience in risk management, and they probably have some idea what they're doing in terms of their lending activities."
Schumer wrote his letter to Ronald Rosenfeld, chairman of the Federal Housing Finance Board, which oversees the system of 12 regional banks that offer financing to mortgage lenders.
Countrywide did its borrowing from the FHLB's Atlanta bank.
As defaults mounted and capital markets tightened, Countrywide shifted its business to emphasize smaller, safer home loans that Freddie Mac and Fannie Mae will buy.
But concerns about the government-sponsored enterprises grew last week after Freddie Mac posted a larger-than-expected $2 billion third-quarter loss, and said it was looking to bolster capital. If GSEs were constrained from buying so-called "conforming" home loans, Countrywide's business could suffer.
Yet Bigelow said: "Countrywide does not believe that Freddie Mac's third-quarter loss or any other recent news from the GSEs will have a material impact on our ability to fund loans."
Countrywide funded $22 billion of mortgage loans in October, down 48 percent from a year earlier.