Just some more detail from the WSJ: Mortgage Securities Back Fed Loan to Bear Stearns

The securities backing a $29 billion Federal Reserve loan to Bear Stearns Cos. consist primarily of "mortgage-backed securities and related hedge investments," the Treasury Department said.

...

The Fed has declined to provide any underlying detail so far.

JPMorgan will take the first $1 billion in losses on the $30 billion portfolio, and the U.S. taxpayers will pay for the remaining losses (if any).