ACA Financial Guaranty Corp. sued billionaire hedge-fund manager John Paulson's firm for allegedly inducing the company to insure part of a mortgage-linked investment that soured.

The amended complaint, filed by the bond insurer in New York state court Wednesday, adds Paulson & Co. and its Credit Opportunities Master II Ltd. fund as defendants in a suit ACA filed in January 2011 against Paulson's banker, () Inc. (GS).