Kayla Mueller: Photos of Captured Activist

In August 2013, American human rights activist Kayla Mueller was taken captive by ISIS in Aleppo, Syria, after leaving a Doctors Without Borders hospital. She remained a hostage for 18 months before being killed in a Jordanian airstrike. Tonight, ABC News will discuss her parents desperate efforts to negotiate for their daughter's freedom, and will speak with other former fellow hostages, who will describe Kayla's bravery despite the inhumane conditions they were subject to in captivity. Click through our gallery to see photos of Kayla Mueller. (Twitter/ABCNews)