Activist investor Carl Icahn on May 11 added two more nominees to his slate for the board of SandRidge Energy Inc. (NYSE: SD) after the company expanded its board, suggesting that the hedge fund manager planned to press ahead with the proxy fight.

The U.S. shale oil producer said last week it would expand the board size to seven from five, to allow investors to pick two nominees from Icahn's five-person slate.

RELATED: Carl Icahn Nominates Five To Replace Entire SandRidge Board

Icahn, who holds a 13.51% stake in SandRidge, got the company to back out of its planned buyout of rival Bonanza Creek Energy Inc. (NYSE: BCEI) and forced the removal of the company's CEO and CFO.

The billionaire investor on May 11 nominated Jonathan Christodoro, a former managing director of Icahn Capital LP, and Nancy Dunlap, the private counsel and head of the private family office of former U.S. Senator Jon Corzine.