Maurice R. "Hank" Greenberg, the longtime former chief executive of American International Group Inc., launched a legal challenge to the government's 2008 takeover of AIG, alleging the move was unconstitutional.

Mr. Greenberg's lawyers on Monday sued the U.S. and the Federal Reserve Bank of New York on behalf of Starr International Co. and other AIG shareholders. The lawsuits accused the Treasury Department and New York Fed of wrongly taking control of the insurer and using it as a vehicle to funnel tens of billions of dollars...