Doctors Without Borders Refused to Help American ISIS Hostage Kayla Mueller

Even though she was kidnapped by ISIS from a Doctors Without Borders vehicle, and was helping at a Doctors Without Borders hospital in Syria, the prestigious humanitarian group refused to help negotiate for the freedom of American hostage Kayla Mueller, her parents tell ABC News.

Marsha and Carl Mueller of Prescott, Arizona, say the group refused to speak with them for months and then withheld critical information brought out of by freed Doctors Without Borders hostages -- information that directly concerned their daughter and was needed in order to begin negotiations for her release.

In a phone conversation recorded by the Muellers ten months after their daughter's kidnapping and provided to ABC News, the parents asked the group if they would help negotiate for their daughter. "No," the senior official replied. "So, the crisis management team that we have installed for our five people -- and that managed the case for our people -- will be closed down in the next week. Yeah? ... Because our case is closed."

"They're a fabulous organization and they do wonderful work," Carl Mueller told ABC News' "20/20" in an interview to be broadcast this Friday, "but somewhere in a board room, they decided to leave our daughter there to be tortured and raped and ultimately murdered."

Tune in to ABC News "20/20" Friday, Aug. 26, at 10 p.m. ET for the full Brian Ross report, "The Girl Left Behind."

Mueller was killed in ISIS captivity in February 2015, 18 months after she was taken. She was kidnapped as she accompanied her boyfriend to Syria to help install communications equipment for Doctors Without Borders, also known overseas by its French acronym, MSF.

The organization's top American official said the group had no obligation to help the young American woman.

"I don't think there was a moral responsibility," Jason Cone, the U.S. executive director, said in an interview with ABC News. "We can't be in the position of negotiating for people who don't work for us."

Cone said Mueller had not been asked by the group to come to Syria and would have not have permitted her to travel there if it had been asked because of her American citizenship.

At least seven staff members of Doctors Without Borders were released by ISIS, including after the group helped to negotiate ransom payments. But the group refused to include Mueller in the negotiations, or to speak with the FBI case agent handling her case, according to an April 2014 email from a senior Doctors Without Borders official in Brussels, provided to ABC News by the family.

Chris Voss, a retired FBI chief hostage negotiator, said he found the humanitarian group's decision not to aid Kayla or her family "stunning."

"I think that's totally abandoning someone that you had no reason to abandon. I mean, it sets that person up for incredibly negative, horrific consequences," Voss said in the "20/20" investigation, which airs on Friday. "They could've said, 'Yes, you work for us.' And they could've extended her some sort of protection, some sort of legitimacy that would've cost them nothing. And why they leave her out there like that -- it's frightening, it scares me."

"It's a lack of appreciation for another human being," Voss added.

A Doctors Without Borders press release at the time of Mueller's reported death on Feb. 6, 2015, stated: "Despite media reports, MSF wishes to clarify that at no time was Ms. Mueller employed by MSF, either in Syria or anywhere else."

The group recently deleted the statement from its website, at the request of the Muellers, because "it was insensitive at a time of incredible grief for them," Cone told ABC News.

The young American's horror began with what seemed to be a safe trip from Antakya, Turkey, to Aleppo, Syria, to help her friend Omar Alkhani, a Doctors Without Border contractor, on Aug. 3, 2013. Kayla helped him install a satellite internet device at a hospital run by the group's Spanish affiliate, according to her family, friends and the FBI. It was late in the day, so the hospital advised they stay overnight. The next day, Aug. 4, Kayla and Omar finished the work and hospital staff put them in an MSF-marked, "locally hired" vehicle to be transported to the Aleppo bus station to return across the Turkish border.

But they never made it.

A large group of ISIS gunmen stopped the vehicle and took prisoner Kayla, Omar, the hospital's assistant logistics manager and the driver. All were Syrian except Kayla and all were released within weeks -- except the young American.

When three female staff members kidnapped later, in January 2014, were finally released in April 2014 by ISIS, they had been told to memorize an ISIS email address to give to Kayla's parents -- but Doctors Without Borders officials failed to tell the Muellers about that for at least seven weeks, until the final two male members of its staff were also set free by ISIS.

The delay in handing over the email address probably angered ISIS, said Voss, the former FBI hostage negotiator who once oversaw hostage recovery operations in Iraq.

"Looking at the situation from the adversary's point of view, the Da'esh kidnappers, they can't imagine this would happen. This makes no sense to them. They have to be frustrated by this. They have to be kinda confused by it," Voss said, using an alternative term for ISIS.

ABC News' Rhonda Schwartz, Lee Ferran, Matt McGarry, Engin Bass, Mark Dorian, Alex Hosenball and Cho Park contributed to this report.