Citadel investment Group LLC raised its stake in E*Trade Financial Corp. to 17 percent after purchasing 90.9 million shares for about $100 million, E*Trade announced today.

Citadel, Chicago-based diversified financial investment and trading firm, had pledged to buy between 50 million and 100 million shares when E*Trade announced a stock and debt exchange offer on Wednesday.

Last week, Citadel’s chief executive officer and founder Kenneth C. Griffin, 40, was added to the company’s board of directors, exercising a right under a 2007 agreement.