Defense Secretary Ash Carter on Thursday removed his senior military assistant, Army Lt. Gen. Ron Lewis, after "learning about allegations of misconduct," the Pentagon said in a statement.

It did not specify what the alleged misconduct may have been, but NPR's Tom Bowman reports Lewis is accused of an improper personal relationship.

"I expect the highest possible standards of conduct from the men and women in this department, particularly from those serving in the most senior positions," Carter said in the statement. "There is no exception."

Lewis, a longtime Carter aide, previously worked for him when Carter was deputy secretary of defense under Leon Panetta and Chuck Hagel, and before that when Carter was the Pentagon's top weapons buyer. Lewis also served as chief of Army Public Affairs and prior to that as a top commander with the 101st Airborne Division (Air Assault) based at Fort Campbell, Ky.

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