Moody’s is reviewing the Aa3 rankings of Assured Guaranty Municipal Corp., Assured Guaranty Corp. and their affiliated insurance operating companies, the New York-based ratings company said in a statement Tuesday. Standard & Poor’s lowered Assured ratings two steps in November to AA-, a level that’s equivalent to Moody’s current ratings. The shares fell 13.4 percent to $16.30, the biggest drop since Aug. 8, according to data compiled by Bloomberg.

“We are surprised that Moody’s decided it had enough information to place Assured Guaranty on review for possible downgrade” because the company has been working with the ratings firm and the review process isn’t complete, Chief Executive Officer Dominic Frederico said in a statement Tuesday. “Moody’s action was unjustified and unwarranted.”

Moody’s cited lower origination volume and reduced demand for the Hamilton, Bermuda-based bond insurer’s business, economic stress in the U.S. and Europe related to mortgage and municipal finance and pressure on operating margins with interest rates at about record lows.

Assured has recorded record operating income since the second quarter of 2009, the last time Moody’s put the insurer on review, with return on equity of 14.9 percent in 2010 and 12.1 percent in 2011, Frederico said in the statement. It has insured more than $58 billion of U.S. municipal bonds since that time and captured one out of every eight new issues last year, he said.