Mueller was reportedly held for some time by Sayyaf and his wife, Umm Sayyaf, who was also captured by US special forces in May.
At the time, the Pentagon said Umm Sayyaf was suspected of being an IS member and of being complicit in the enslavement of a young Yazidi woman who was rescued in the raid.
Hundreds of young women and girls - many of them Yazidis captured in northern Iraq - are believed to be held as sex slaves by IS militants in areas under their control.
The Yazidi girls provided intelligence used by the US to interrogate Sayyaf’s wife, who “spilled everything” about several IS leaders and their whereabouts, a counterterrorism official told ABC.
Umm Sayyaf was handed over to the Kurdish authorities in northern Iraq last week to face trial.
The information that has come to light appears to contradict speculation that Mueller was treated well in captivity, as a letter written in 2014 and smuggled out to her family implied.
In it, Mueller tried to reassure her family, saying that she had been treated with “utmost respect + kindness”.
The humanitarian aid worker from Prescott, Arizona, travelled to the Turkey-Syria border in 2012 to work with refugees.
http://www.bbc.com/news/world-us-canada-33941913