The widow of late Daesh group financial leader Abu Sayyaf was charged on Monday for her alleged role in the death of US aid worker Kayla Mueller last year.
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 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.
Daesh 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.
US officials say the circumstances of her death remain unclear. She was 26.
Abu Sayyaf was killed in May 2015 in a rare US commando raid inside war-torn Syria.
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.
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,” 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."
The complaint also alleged that Bahar told the captives that "she would kill them if they did not listen to her."
Bahar admitted that she had sole responsibility for holding the hostages captive while her husband travelled, 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.