'20/20' Investigation Reveals Shocking Details About the Captivity and Fight for Freedom of American ISIS Hostage Kayla Mueller

Report Features Exclusive Interviews With Fellow Hostages, Never Before Seen Communications With ISIS and Kayla's Proof-of-Life Video

'The Girl Left Behind' Airs Friday, August 26, on ABC

When 26-year-old Kayla Mueller, a humanitarian aid worker from Prescott, Arizona, was kidnapped by ISIS in 2013, it set off a desperate effort by her parents to gain her freedom, dealing not only with one of the most brutal terror groups the world has ever known, but also with humanitarian groups and U.S. officials who they say failed their daughter.

In an eye-opening report, ABC News Chief Investigative Correspondent Brian Ross and the Investigative Team expose shocking details about the attempted and failed efforts to free Mueller. This hour-long special includes never before seen videos and voice messages sent by the young woman in captivity, the lengthy communication with ISIS and Kayla's parents, Carl and Marsha Mueller, and exclusive interviews with hostages who were imprisoned with Kayla. "The Girl Left Behind" airs on "20/20," FRIDAY, AUGUST 26 (10:00-11:00 p.m. EDT), on the ABC Television Network.

Ross sits down with Carl and Marsha as they share, for the first time, the full details of their tireless efforts to rescue Kayla. The Muellers reveal to Ross the emails they exchanged directly with ISIS, attempting to negotiate for their daughter's life and the proof-of-life video sent to them of Kayla begging for help. They explain their frustration with the U.S. government who kept Kayla's captivity a secret, threatening prosecution if they attempted to pay the ransom, and a deep disappointment with President Obama who originally refused to meet them.

Kayla's former fellow hostages, Frida Saide and Patricia Chavez, speak exclusively with "20/20," describing her life in captivity and the inhumane conditions and abuse they all experienced while being held by ISIS. Daniel Rye, Danish freelance photographer, held captive in the same complex as Kayla, tells how she stood up to Jihadi John in front of the male captives. Another hostage, a 13-year-old member of the Yazidi tribe also held captive with Kayla, shares how when she and a fellow hostage told Kayla of their plans to escape, Kayla insisted they leave without her because her presence would make it too dangerous.

"20/20" is anchored by Elizabeth Vargas and David Muir. David Sloan is senior executive producer.