Fannie Mae challenged Bank of America Corp.'s assertion that the two decided by mutual consent in January not to renew their loan-purchase agreement in the middle of a battle over who should pick up the multibillion-dollar tab for bad mortgages.

Rather the mortgage-finance company said in interviews and filings Wednesday that it alone opted against renewing its contract with Bank of America because the bank was resisting demands to buy back defaulted mortgages. "We felt like we had to take that step," Susan McFarland, Fannie's...