WASHINGTON (AP) — Orders to U.S. factories fell in May by the largest amount in three months, while a key category that signals business investment plans dropped for a second month.

The Commerce Department says factory orders declined 1 percent in May from April, when orders retreated 0.7 percent. Orders in a category that serves as a proxy for business investment were down 0.4 percent after a 0.7 percent decline in April.

Much of the weakness in May reflected a big 35.3 percent fall in demand for commercial aircraft.

But even outside of the volatile transportation category, orders were up only a tiny 0.1 percent. The weak showing suggests that manufacturing is still struggling with challenges such as lower energy prices and a strong dollar, which dampens exports.