Assets the Federal Reserve agreed to accept in March from Bear Stearns Cos. to facilitate its sale to J.P. Morgan Chase & Co. are now valued about $1 billion less than estimated at the time of the transaction.

The decline, to $28.9 billion from $30 billion in mid-March, isn't yet enough to put U.S. taxpayers on the hook for any losses; J.P. Morgan, under an agreement with the Fed, would take the first hit once the assets are sold. In addition, the assets are expected to be sold over the coming decade in a way that is designed to minimize losses and market disruption.

Late last month, the Fed contributed $28.8 billion to a new firm, Maiden Lane LLC, to finance the assets and provide a loan. J.P. Morgan supplied $1.15 billion, which would cover the first losses from any decline in the assets' value. Maiden Lane is a street running alongside the New York Fed's headquarters.

The Fed agreed to extend the loan in March with Bear Stearns on the brink of bankruptcy. Central-bank officials feared that the sudden downfall of the fifth-largest U.S. investment bank would wreak havoc on broader financial markets and the economy.

The Bear Stearns portfolio at the time largely contained mortgage-related assets, which are riskier than other investments, but didn't include subprime mortgage debt. It was originally valued in part by Bear Stearns, while the recent valuation was conducted by the Fed and its adviser, BlackRock Inc. The Fed plans to release an updated value of the portfolio quarterly.