The Federal Open Market Committee left interest rates unchanged Wednesday and said it would buy $400 billion of Treasury bonds in an effort to lower long-term borrowing costs. The bond buying program begins Oct. 3. The committee also said “to help support conditions in mortgage markets” it will reinvest principal payments from agency debt into agency mortgage-backed securities. That’s a departure from the Federal Reserve’s previous practice of reinvesting those proceeds into Treasurys and appears to be an effort to lower mortgage rates. The purchase program, to be completed by the end of June, will involve longer-term Treasury securities with remaining maturities of six years to 30 years, and will be financed through the sale of shorter-term Treasurys with maturities of three years or less. “This program should put downward pressure on longer-term interest rates and help make broader financial conditions more accommodative,” the FOMC said in a statement following its two-day meeting. “I don’t think this will make a material difference to the economic outlook,” said Paul Dales, senior U.S. economist with Capital Economics, a consultancy firm based in Toronto. “It might help at the margin, but the problem isn’t really the price of credit, it’s the inability of households to borrow and the unwillingness of companies to invest, so I don’t think this is by any means going to materially alter the situation,” Dales said. The FOMC also kept the target range for the federal funds rate at zero to 0.25% and said, in language that echoed its August statement, it “currently anticipates that economic conditions — including low rates of resource utilization and a subdued outlook for inflation over the medium run — are likely to warrant exceptionally low levels for the federal funds rate at least through mid-2013.” The move to change the composition of the Fed’s balance sheet has been dubbed Operation Twist after a similar effort undertaken during President Kennedy’s administration, when the dance craze was sweeping the nation. A study by Federal Reserve Bank of San Francisco economists in April found evidence the federal effort in 1961 had a moderate effect on the nation’s economy. “Four of six potentially market-moving Operation Twist announcements had statistically significant effects and that the program cumulatively caused a significant but moderate 0.15 percentage point reduction in longer-term Treasury yields,” Titan Alon and Eric Swanson of the San Francisco Fed said. Three Fed policymakers dissented from the committee’s decision because they “did not support additional policy accommodation at this time,” said the statement. They were: Richard Fisher, president of the Dallas Federal Reserve Bank, Narayana Kocherlakota, president of the Minneapolis Fed and Charles Plosser, president of the Philadelphia Fed. Write to Liz Enochs.