Nisreen Assad Ibrahim Bahar, 25, an Iraqi citizen known as Umm Sayyaf, is the widow of Abu Sayyaf, who was a senior leader in the terror group until his death last year.
She and was charged by criminal complaint for her role in a conspiracy that resulted in the death of Mueller in February last year, the Justice Department announced on Monday.
The humanitarian worker, from Arizona, was killed in Syria after being held captive by the ISIS for almost two years. It is still not clear whether her death was at the hands of her captors or as a result of a coalition airstrike.
The criminal complaint accuses Sayyaf and her husband of holding Mueller and other women captive.
She is now in custody in Iraq due to her terror-related activities, the department said. If convicted, she faces life in prison.
U.S. intelligence officials have told Mueller's family that their daughter was forced to have sex with Abu Bakr al-Baghdadi, the leader of the group.
Abu Sayyaf, who reported directly to al-Baghdadi, was the terror group’s minister of oil and gas, and previously responsible for their media program.
He was killed in a Delta Force raid of his Syrian compound in June, and his wife was turned over to Iraqi authorities for prosecution.
According to the affidavit in support of the criminal complaint, Mueller and another person were kidnapped at gunpoint by masked ISIS gunmen on their second full day in northern Syria in 2013.
In August 2014, two young Kurdish women of Yazidi heritage were taken from their village in northern Iraq and were taken to a prison in Syria where they were held alongside Mueller.
Then, around September 24, 2014, Mueller and the other captives were taken from the ISIS prison and transferred into the custody of Abu and Umm Sayyaf.
According to the Justice Department, the Sayyafs maintained several residences where they held Mueller and a number of other female captives.
Umm Sayyaf was interviewed by FBI agents in June last year, where she admitted that she knew how al-Baghdadi treated Mueller.
She told agents that al-Baghdadi ‘owned’ Mueller during her captivity and also said that ‘owning’ is equivalent to slavery, the affidavit added.
She also admitted that her family belong to al-Qaeda in Iraq and remained members of the organization when its name was changed to ISIS.
Sayyaf admitted to holding hostages on behalf of the group and said she had sole responsibility for Mueller and others while her husband traveled on ISIS business.
She said she would threaten the women, and told them she would kill them if they did not listen to her.
Their prisoners were held in locked rooms and handcuffed at various times, Daily Mail reported.
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