The Dow Jones industrial average closed down more than 500 points on Wednesday, nearly 5 percent.
The tumble comes a day after the market rallied, closing up more than 400 points on Tuesday. Still, the fall comes amid market volatility and heavy volume, with the index dropping significantly after Standard & Poor's downgrade of U.S. credit worthiness.
The Nasdaq lost about 4 percent; while the S&P 500 fell nearly 4.5 percent.
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