The Federal Reserve is open to selling some of the securities now on its books as part of its withdrawal from its unconventional efforts to prop up the economy, Chairman Ben Bernanke said Thursday, in a change of tone on how the Fed will execute its exit strategy from crisis-era interventions. Over the past 15 months, with the Fed's short-term interest rate target near zero and the economy in horrible shape, the central bank bought more than $1.7trn in long-term assets to push rates down further. The purchases of those assets -- including mortgage-backed securities, debt in companies like Fannie Mae and Freddie Mac, and long-term Treasury bonds -- helped swell the Fed's balance sheet from about $800bn before the crisis to $2.3trn last week. Bernanke, testifying Thursday before the House Financial Services Committee, said that "if necessary," the Fed "has the option of redeeming or selling securities" bought during the crisis.