Umm Sayyaf, wife of the Islamic State's head of oil operations Abu Sayyaf, was in U.S. custody since May 15.
There have been suggestions that she and her husband were the captors of American hostage Kayla Mueller. Mueller was killed while being held prisoner. Islamic State militants have claimed she died in a Jordanian airstrike.
U.S. officials have said Umm Sayyaf was cooperative and provided a "trove" of intelligence about Islamic State operations.
