Dow Jones Industrial Average Closes Flat After ECB Turns Up The Heat On Greece

U.S. stocks closed lower Wednesday, with the blue-chip Dow swinging between negative and positive territory in the last hour of trading, after the European Central Bank said it is blocking banks from using Greek debt as collateral, increasing investors' concerns about the global economy. Meanwhile, crude-oil prices closed lower, snapping a four-day rally, after U.S. oil inventories surged to an 80-year high.