U.S. mortgage rates fell more than three-quarters of a percentage point Tuesday after the Federal Reserve said it will buy as much as $600 billion of debt.

The average U.S. rate for a 30-year fixed mortgage ended the day at about 5.5 percent after falling to as low as 5.25 percent, according to Bankrate Inc. It was 6.38 percent Tuesday morning, according to Bankrate, which is based in North Palm Beach, Fla., based on a wider sampling than the so-called overnight rate published on its Web site.

The Fed said it will purchase mortgage-backed securities issued by Fannie Mae, Freddie Mac and Ginnie Mae, a government agency that insures bonds. Tuesday's lower rates indicate the central bankers may have achieved their goal of bringing liquidity to mortgage markets, said Neal Soss, chief economist at Credit Suisse Group in New York and a former aide to Fed chief Paul Volcker.