American aid worker Kayla Mueller was only 24 years old when the so-called Islamic State (IS) took her hostage in 2013.
She had travelled to the Syria-Turkey border to help refugees.
After 18 months in captivity, IS claimed she had been killed by a Jordanian airstrike, but the US maintains she died at the hands of the terrorist group.
With no definitive proof about what happened to Kayla, her family is still carrying out their own search for answers.
UK viewers can watch
In The Face of Terror on Monday 5 October at 21:00 BST on BBC Two and on BBC iPlayer.