The Federal Reserve on Tuesday said that the risk of a downturn in the nation’s economy had increased, and that it was prepared to use additional policy tools, including extending its period of exceptionally low interest rates, until at least 2013.

The Fed’s announcement was eagerly awaited by investors who have responded to grim economic tidings in recent weeks by driving down global markets.

The economy grew only 0.8 percent during the first half of the year. The work force is shrinking. State and local governments are cutting back. And fiscal policy is immobilized by partisanship, leading Standard & Poor’s to remove the United States from its list of risk-free borrowers.

That has left investors to hope that the Fed would consider new steps to help the economy.

The central bank has held its benchmark short-term interest rate near zero since December 2008, flooding the financial system with the nearest thing to free money. It has promised after each of its meetings since late 2008 to keep interest rates near zero “for an extended period,” which Mr. Bernanke defined earlier this year as meaning a period of at least several months.

The central bank also has amassed more than $2.5 trillion in Treasury securities and mortgage-backed securities, putting downward pressure on long-term interest rates. The purchases have pushed investors into the stock market and other riskier investments, and reduced the value of the dollar, helping American exporters. The Fed has said that selling off these assets would be its first step when the economy begins to improve, but it has avoided setting any timetable for a wind-down.

Mr. Bernanke said last month that the Fed was “prepared to take further steps if needed,” but he made clear that the central bank was reluctant to do so. He said the Fed would act only if growth continued to falter and, importantly, only if price increases slowed, stopped or reversed.

The inflation of prices and wages is the Fed’s primary concern. By law the Fed is responsible for keeping prices steady and unemployment as low as possible. But Mr. Bernanke, like his predecessors, places greater emphasis on prices, in part because the Fed has concluded that slow, steady inflation — about 2 percent a year — is the best atmosphere for enduring job growth.

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