LOS ANGELES (AP) -- The U.S. regulator of credit unions on Tuesday sued Goldman Sachs & Co. for more than $491 million in damages over losses incurred by two failed credit unions that purchased mortgage-backed securities underwritten by the investment bank.The complaint filed by the National Credit Union Administration in U.S. District Court in Los Angeles is the latest lawsuit brought by the federal regulatory agency against a major bank as it seeks to recover billions in losses related to risky mortgage-backed securities that brought down credit unions in recent years.Buyers of mortgage-backed securities, mostly banks, pension funds and other big investors, made money from the investments if the underlying debt was paid off. But as U.S. homeowners started falling behind on their mortgages and defaulted in droves in 2007, the securities failed and their buyers lost billions.In the complaint, which also names as defendants several issuers of mortgage-backed securities, regulators claim that the documents used in offering the securities contained untrue statements or omissions as to how risky the investments were.As a result, U.S. Central Federal Credit Union in Lenexa, Kan., and Western Corporate Federal Credit Union in San Dimas, Calif., acquired the mortgage-backed securities, believing the risk of loss was minimal, according to the complaint.However, even though virtually all of the securities had a triple-A rating, they represented a substantial risk of losses, the NCUA claims. And when the investments' market value plummeted, the credit unions -- two of the nation's largest -- failed.The NCUA placed the two credit unions into conservatorship in March 2009. In October of 2010, it placed them into involuntary liquidation.Goldman Sachs declined to comment Tuesday.The NCUA says it may sue five to 10 other banks in coming weeks. In June, regulators sued JPMorgan Chase & Co. and Royal Bank of Scotland PLC.Factoring in the latest lawsuit, regulators are seeking to recover nearly $2 billion in damages.Any recoveries from the lawsuits would reduce the total losses resulting from the failure of Western Corporate, U.S. Central and three other failed corporate credit unions: Southwest Corporate, Members United Corporate and Constitution Corporate, the NCUA said.Corporate credit unions provide financing and investment services to the much larger population of retail credit unions.Shares of The Goldman Sachs Group Inc. added 50 cents to $123.30 in aftermarket trading. The shares ended the regular trading session up $5.07, or 4.3 percent, to $122.73.