The Pentagon says its second-ranking official is stepping down but has not said why.

In a written statement Thursday, Defense Secretary Chuck Hagel said he met with his deputy, Ashton Carter, and accepted his decision to leave on Dec. 4.

The 59-year-old Carter became the deputy defense secretary in October 2011 when Leon Panetta was the Pentagon chief. Before that he served as the head of acquisition, technology and logistics. He also served in the Pentagon in the mid-1990s.

Hagel said Carter has an "unparalleled knowledge of every facet" of the U.S. defense system and will be missed.