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 <.DJI> slid 185.68 points, or 1.96 percent, to end unofficially at 9,310.52. The Standard & Poor's 500 Index <.SPX> lost 22.58 points, or 2.21 percent, to finish unofficially at 998.04. The Nasdaq Composite Index <.IXIC> dropped 40.17 points, or 2.00 percent, to close unofficially at 1,968.89.

(Reporting by Leah Schnurr; Editing by Jan Paschal)