Kayla Mueller's parents: 'We can't let her down again'

After being taken hostage by the so-called Islamic State in 2013, no one knows for certain what happened to American aid worker Kayla Mueller.

Kayla Mueller's parents: 'We can't let her down again'
After being taken hostage by the so-called Islamic State in 2013, no one knows for certain what happened to American aid worker Kayla Mueller.