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September's prices were down in all 20 cities, compared to both August 2008 and September 2007, the data showed.

The largest decline in September was found in the San Francisco metropolitan area, where prices fell 3.9%. In the past year, Phoenix has had the largest decline -- 31.9%.

For the original 10-city index, prices declined a record 18.6% in the previous 12 months. For the third quarter, there was a record year-over-year decline of 16.6% in the national home price index.

"The turmoil in the financial markets is placing further downward pressure on a housing market already weakened by its own fundamentals," said David Blitzer, chairman of the index committee at Standard & Poor's.

All three Case-Shiller aggregate indexes, as well as 13 of the 20 metro areas, saw new record rates of decline, Blitzer noted. (About the credit’s problems see The credit crunch has dramatically increased the potential returns)

Ian Shepherdson, chief U.S. economist with High Frequency Economics, wrote in a note that it seems "a bit early" to observe the effects of market chaos in the home price data.

"When the credit problems first began in July last year, home prices did not clearly respond until November," Shepherdson wrote. "Perhaps the immediate story is that the proportion of foreclosure sales at very low prices is rising."

On a related note, the Federal Reserve announced Tuesday a plan to help support housing markets.

The U.S. central bank said it would be purchasing up to $100 billion in direct obligations of housing-related government-sponsored enterprises, such as Fannie Mae, Freddie Mac and the Federal Home Loan Banks, as well as up to $500 billion in mortgage-backed securities backed by Fannie, Freddie and Ginnie Mae.

The move will help reduce the cost and increase the availability of credit for home buyers, the Fed said, adding that this in turn would help support the housing market. See related story.

Here's how prices in the 20 cities performed in the past year:

Phoenix, down 31.9%; Las Vegas, down 31.3%; Miami, down 28.4%; San Francisco, down 29.5%; Los Angeles, down 27.6%; San Diego, down 26.3%; Detroit, down 18.6%; Tampa, down 18.5%, Washington, down 17.2%; Minneapolis, down 14.4%; Chicago, down 10.1%; Seattle, down 9.8%; Atlanta, down 9.5%; Portland, down 8.6%; New York, down 7.3%; Cleveland, down 6.4%; Boston, down 5.7%; Denver, down 5.4%; Charlotte, down 3.5%; Dallas, down 2.7%.

Ruth Mantell, MarketWatch