NEW YORK, Sept 1 (Reuters) - U.S. stocks fell on Tuesday, pushing the S&P 500 below the 1,000 threshold, as uncertainty over financial companies' health prompted a sell-off, while concerns mounted about the validity of a rally that has driven stocks up about 50 percent from March.
* The Dow Jones industrial average slid 185.68 points, or 1.96 percent, to end unofficially at 9,310.60. The Standard & Poor's 500 Index lost 22.58 points, or 2.21 percent, to finish unofficially at 998.04. The Nasdaq Composite Index dropped 40.17 points, or 2.00 percent, to close unofficially at 1,968.89. (Reporting by Leah Schnurr; Editing by Jan Paschal)