American female hostage ISIS
Kidnapped American aid worker Kayla Mueller may have been married off to one of ISIS fighters, officials have said.Twitter

American aid worker Kayla Mueller, who was kidnapped by Islamic State (ISIS) militants in August 2013 and was confirmed dead by US President Barack Obama on Tuesday, was possibly married off to one of the commanders of the jihadist group during her captivity, officials have said.

The Sunni militant outfit, which has vowed to establish an Islamic Caliphate in Iraq and Syria, had announced last Friday that the aid worker was killed in a Jordanian airstrike in northern Syria. The strike allegedly hit an ISIS hideout where she was being held.  

Obama confirmed the 26-year-old captive's death, although he stressed that there was no evidence to suggest that she was killed during the airstrike, as claimed by the ISIS.

New details have emerged since then on Mueller's captivity, with US counter-terrorism officials suggesting that she was forcibly married off to one of the ISIS militants – a practice that is understood to be rampant in the group.

"ISIS didn't see her as a hostage or a bargaining chip," an unnamed official told ABC News, adding that intelligence gathered on Mueller's whereabouts throughout the period of her captivity suggested she was sometimes seen in the company of an ISIS leader who reportedly was responsible for her custody. This indicates that she might have been forced to marry the leader. 

Intelligence gathered using techniques such as eavesdropping on ISIS members who referred to her during phone conversations, and tracking fighters guarding a sensitive site hinted that that she was with her 'ISIS keeper' most of the time, two officials were cited as saying.

Mueller's family told the media last Friday that the members of the outfit had referred to their daughter as a "guest" during private communications.

The US intelligence, which reportedly has no resources to gather ground-level intelligence from Syria, has to rely heavily on drones and satellites for information. 

US officials confirmed that they were careful not to bombard any location where Mueller was likely to be held, various reports said on Tuesday.

Over the months, reports have suggested that ISIS militants usually sell off or put young women and girls in the company of militants, who treat them like sex slaves.

Officials have also confirmed that the militant group last August had asked for a $6.6 million ransom and threatened to kill her if it was not paid. But the deadline passed without any incident – an indication that she was possibly alive, as the wife of an influential leader and no longer a hostage who deserved to be killed.

In a letter written last year, the 26-year-old had said that she was kept "in a safe location, completely unharmed (and) healthy."

"I have been treated with the utmost respect and kindness," she wrote, as confirmed by her family last week.

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