NEW YORK (Reuters) - The Justice Department has opened an investigation of possible accounting fraud at Fannie Mae, the Wall Street Journal reported on Thursday, after a federal regulator said the mortgage giant may have manipulated its earnings targets. The Journal reported that the investigation is still in the preliminary stages. Fannie has been under fire since the Office of Federal Housing Enterprise Oversight, or Ofheo, accused the company of improper accounting that allowed senior executives to pocket multimillion-dollar bonuses. According to the newspaper, officials have said that some Fannie Mae executives may have misled regulators, which in some cases...