CHICAGO, June 25 (Xinhua) -- The Chicago Fed National Activity Index (CFNAI) fell to negative 0.15 in May from positive 0.42 in April, pointing to slower U.S. economic growth in May, the Chicago Federal Reserve Bank said on Monday.

The CFNAI is a weighted average of 85 existing monthly indicators of national economic activity. A positive index reading corresponds to growth above trend and a negative index reading indicates growth below trend.

Chicago Fed said the negative index was mainly led by declines in production-related indicators, which contributed negative 0.29 to the CFNAI in May, down from positive 0.33 in April.

Manufacturing industrial production decreased 0.7 percent in May after increasing 0.6 percent in April. In contrast, the sales, orders, and inventories category made a contribution of positive 0.05 to the CFNAI in May, up from a neutral contribution in April.

Meanwhile, the index's less-volatile, three-month moving average decreased to positive 0.19 in May from positive 0.48 in April, said Chicago Fed.