The U.S. economy, fueled by an export-driven manufacturing boom, expanded at a robust 3.6 percent in the first three months of this year, slightly less than previously thought, the government said Thursday.

The Commerce Department said the gross national product, the broadest measure of economic health, grew to a seasonally adjusted annual level of $3.915 trillion in the January-March period, a $34.6 billion increase from three months earlier.