Investors put $240.3 million in Series EE bonds during April, according to data from the U.S. Treasury. Only $184.4 million - about 45% of total sales - was invested in Series I bonds. Usually Series I accounts for over half of each month's sales. The surge in Series EE sales was due to the Treasury's April announcement that Series EE bonds sold after May 1 would have fixed rates. Investors who liked the older adjustable-rate Series EE bonds moved up their purchases. Total Savings Bond sales in April, however, were the slowest they've been this fiscal year. Many I bond investors held off their purchases, expecting and getting higher rates in May.