The federal government has filed a lawsuit against a prominent credit ratings agency, alleging the agency issued inflated ratings that misrepresented the true credit risks of mortgage-backed securities in the boom years leading up to the financial crisis and subsequently cost investors billions. The U.S. Department of Justice alleges Standard and Poor's Ratings Services and its parent company, McGraw-Hill Companies Inc., engaged in a scheme to defraud investors -- many of them federally insured financial institutions -- who purchased products known as residential mortgage-backed securities (RMBS) and collateral debt obligations (CDOs) under repeated assurances by S&P that its ratings of these products were objective, independent and uninfluenced by S&P's relationships with the inv...