Nisreen Assad Ibrahim Bahar, a 25-year-old known as Umm Sayyaf, was accused of conspiring to provide support to the violent extremists, forcibly detaining Mueller and other captives in the couple's homes, where she was sexually assaulted by IS chief Abu Bakr al-Baghdadi.
Bahar acknowledged that Baghdadi "owned" Mueller during her captivity at the Sayyaf residences, describing "owning" as equivalent to slavery, federal prosecutors said yesterday.
IS fighters claimed that Mueller, who was kidnapped in the Syrian city of Aleppo in August 2013, was killed in a February 2015 coalition air strike that buried her in rubble.
Bahar was captured during the operation, and US forces also rescued a young woman from the Yazidi minority and seized a stash of firearms, the complaint recalled.
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Mueller and other female "captives were at various times handcuffed, held in locked rooms and given orders on a daily basis with respect to their activities, movements and liberty," according to a complaint filed in US District Court in Virginia.
"While in captivity, Kayla Jean Mueller was sexually abused by Baghdadi, who forced her to have sex with him," it added. "The defendant (Bahar) knew how Ms Mueller was treated by Baghdadi when Ms Mueller was held against her will in the defendant's home."
Bahar admitted that she had sole responsibility for holding the hostages captive while her husband traveled on IS business, and that Baghdadi and other members of the group would stay at the residence at times, according to the complaint.
If convicted, Bahar faces life in prison. She is currently in Iraqi custody, facing prosecution for terror-related activities.
"We fully support the Iraqi prosecution of Sayyaf and will continue to work with the authorities there to pursue our shared goal of holding Sayyaf accountable for her crimes," Assistant Attorney General John Carlin said in a statement.