Bank of America’s Countrywide mortgage unit has been sued by investors claiming they were victimized in a “massive fraud” when they bought mortgage-backed securities. The lawsuit was filed on Monday in a New York state court by 12 plaintiffs including the TIAA-CREF fund family, New York Life Insurance Co and Dexia Holdings Inc. According to the complaint, the investors bought hundreds of millions of dollars of Countrywide securities from 2005 to 2007 that they thought were “conservative, low-risk investments.”