Five Federal Reserve indexes of regional manufacturing all slumped in December, the first time they¡¯ve fallen in unison since May 2016 and the latest evidence that a pillar of the U.S. economy has started to wobble heading into next year.

The Dallas Fed¡¯s factory index unexpectedly contracted in December, falling to a two-year low of minus 5.1 from 17.6 in November, and the steepest decline since 2013. The district bank covers the oil-producing region of Texas, northern Louisiana and southern New Mexico. Oil prices are down about 40 percent from a nearly four-year high in October.

More than 20 percent of manufacturers said that their outlook worsened in December, according to the regional Fed report released Monday. ¡°Expectations regarding future business conditions remained positive but retreated notably in December,¡± it said.