The father of Kayla Mueller, an an American woman killed while being held captive by ISIS, read a moving letter at his daughter's hometown memorial service Saturday, recounting her discovery that her life's purpose was to ease the suffering of others. "This is my life's work," Carl Mueller quoted his daughter as saying. "But my family is my life." Mueller, 26, was an international aid worker who was captured in 2013 after leaving a Doctors Without Borders hospital in Aleppo, Syria, and held for 18 months. An estimated 500 people attended the memorial, held in a college auditorium in Prescott, the northern Arizona mountain community where Mueller grew up.