Wife of ISIS Figure Charged in American Woman's Death

The U.S. Department of Justice today charged the wife of a top ISIS leader for her role in a "conspiracy" that led to the death of American aid worker Kayla Mueller, who was reported killed by the terror group in Syria a year ago.

It was unclear Monday night whether the U.S. was expecting to take custody of Nasrin As'ad Ibrahim, known as "Umm Sayyaf." American forces had originally captured Sayyaf, who is accused of being the "sole" individual "responsible" for Mueller, in a May 2015 but was handed her over to the Kurdish government in August.

Sayyaf was the wife of ISIS's oil and gas "emir," or chief, Abu Sayyaf, a Tunisian who U.S. officials say oversaw the terror group's sales of illicit fossil fuels to fund the ISIS war machine.

Kayla Mueller, 26, of Prescott, Arizona, was a committed humanitarian aid worker captured in Aleppo, Syria, and held for 17 months as a hostage with other Westerners. In the fall of 2014, she was personally selected by ISIS "Caliph" Abu Bakr al-Baghdadi to be his personal hostage against her will, family and counter-terrorism sources have said.

For several months, Mueller was believed held by the Sayyafs in their homes in Syria and visited by Baghdadi, who repeatedly raped her, her anguished parents Carl and Marsha Mueller said.

"We were told Kayla was tortured, that she was the property of al-Baghdadi. We were told that in June by the government," Marsha Mueller told ABC News in August.

The Justice Department today alleged that Umm Sayyaf has admitted to the FBI that al-Baghdadi "owned" Mueller during her captivity at the Sayyaf compound and "admitted that 'owning' is equivalent to slavery."

Mueller was captured in a vehicle on a road in Aleppo, which the humanitarian medical group Medicines San Frontiers (Doctors Without Borders) has said happened while she was traveling with several of their staff, one of whom was an MSF contractor who had asked her to assist him in a trip to an MSF hospital.

The MSF contractor later tried to rescue her by telling ISIS she was his wife -- but Mueller had already told the terrorists holding her that she was not married and feared the consequences of lying to them, another close confidante of Kayla's told ABC News.

Mueller was held captive with, but at times segregated from, a group of American, British and European hostages held at an old oil refinery site south of ISIS's de facto capital of Raqqa, Syria.

The U.S. Army's elite unit Delta Force attempted a rescue mission in the area in July 2014 but the hostages had been moved just days before the counter-terror squad moved in, U.S. officials said.

One by one, the western hostages were beheaded beginning in August on video by ISIS "executioner" Mohammed Emwazi, dubbed "Jihad John." But Kayla Mueller was never shown on video or publicly threatened.

But in February 2015, ISIS claimed she had been accidentally killed by a Jordanian airstrike. U.S. officials denied that there had even been any Jordanian airstrikes that day, and some vowed to find Abu Sayyaf and bring him to justice in a lower Manhattan federal court where many terrorists have been tried and convicted.

That opportunity came for Delta on May 15 in a ground force operation against a house in Syria, the White House said in an announcement afterward. As Sayyaf's guards tried to hide from the American commandos, they all were killed. The Delta operators then killed Abu Sayyaf "when he engaged U.S. forces," Defense Secretary Ash Carter said,

Umm Sayyaf, his wife, was captured alive and one Yazidi girl was rescued. The wife of the Tunisian senior ISIS leader was grilled for weeks by the FBI-led High Value Interrogation Group and she quickly confirmed that Mueller had been held prisoner in their household for Baghdadi, who had raped her, counter-terrorism sources told ABC News. Some intelligence prior to Umm Sayyaf's interrogation had assessed that Abu Sayyaf held her and had taken the American hostage as his own forced "wife," but it became apparent that he had actually kept her as a captive for his leader.

Last August, the U.S. turned Umm Sayyaf over to the Iraqi Kurdistan Regional Government's Ministry of Interior, the Defense Department announced. "The decision to transfer Umm Sayyaf to the Iraqi government was based on the U.S. government determination that the detainee's transfer would be appropriate with respect to legal, diplomatic, intelligence, security, and law enforcement considerations," the DOD statement said.

"The charges filed today allege that Umm Sayyaf and others conspired to provide material support to ISIL and that this conspiracy resulted in the death of Kayla Jean Mueller," said Assistant Attorney General Carlin said in a statement today. "Sayyaf is currently in Iraqi custody for her terrorism-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. At the same time, these charges reflect that the U.S. justice system remains a powerful tool to bring to bear against those who harm our citizens abroad. We will continue to pursue justice for Kayla and for all American victims of terrorism."