PHOENIX, Arizona (Reuters) - The parents of an American humanitarian worker held hostage by the Islamic State in Iraq and Syria (ISIS) since August 2013 said on Friday that they were still hopeful she is alive, after the group said she was killed in a bombing by Jordanian fighter jets.
In a statement released by a family representative, Mr Carl and Mrs Marsha Mueller of Arizona, parents of Ms Kayla Jean Mueller, asked ISIS to contact the family privately.
"You told us that you treated Kayla as your guest, as your guest her safety and well-being remains your responsibility," they said in a message directed to "those in positions of responsibility for holding Kayla".
ISIS said on Friday that Ms Mueller, 26, was killed when Jordanian fighter jets bombed a building where she was being held, but Jordan expressed doubt about the Islamist militant group's account of her death.
In Washington, US officials said they could not confirm that the woman from Prescott, Arizona, had been killed.
"We are looking into it but our first reaction is that we think it is illogical and we are highly sceptical about it. ... It's part of their criminal propaganda," government spokesman Mohammad Momani said in response to ISIS' account of what happened to Ms Mueller. "How could they identify Jordanian war planes from a huge distance in the sky? What would an American woman be doing in a weapons warehouse?" Mr Momani added.
White House National Security Council spokeswoman Bernadette Meehan said the United States is "deeply concerned" over the report but had not seen "any evidence that corroborates" the group's account.
ISIS, in a message monitored by SITE, said Ms Mueller died when the building in which she was being held outside Raqqa, a stronghold of the group, collapsed in a Jordanian air strike on Friday. "The air assaults were continuous on the same location for more than an hour," ISIS said, according to SITE.
The group released photos of what it said were the building's wreckage but did not include photos of Ms Mueller.
French journalist Nicolas Henin, a former captive of the group in Syria who gained his freedom last April, said on Twitter: "Kayla Mueller was among the very last of my former cellmates still detained. I was full of hope she could have a way out."
The US military last summer carried out an unsuccessful mission to rescue American hostages held by the group in Syria.
Reuters and other Western news organisations were aware Ms Mueller was being held hostage but did not name her at the request of her family members, who believed the militants would harm her if her case received publicity.
Ms Mueller, a 2009 Northern Arizona University graduate, had a long record of volunteering abroad and was moved by the plight of civilians in Syria's civil war. "For as long as I live, I will not let this suffering be normal - something we just accept," Ms Mueller's local newspaper The Daily Courier quoted her in 2013 as saying. "When Syrians hear I'm an American, they ask, 'Where is the world?' All I can do is cry with them, because I don't know," she said.
She had worked for a Turkish aid organization on the Syrian border and volunteered for schools and aid organizations abroad including in the West Bank, Israel and India. "The common thread of Kayla's life has been her quiet leadership and strong desire to serve others," according to a statement from her family's representative.
Ms Mueller felt compelled to help others from an early age, according to a statement from the family. "When asked what kept her going in her mission, she said 'I find God in the suffering eyes reflected in mine, if this is how you are revealed to me, this is how I will forever seek you'," the statement said.
As a high school student at Tri City College Prep, she received several awards, in part for her volunteering with groups like AmeriCorps and Big Brothers Big Sisters, a statement said.
She graduated from Northern Arizona University in 2009 and went on to work for humanitarian aid groups in northern India, Palestine, and Israel before returning to Arizona to work at an HIV/Aids clinic and volunteer at a woman's shelter, it said.
Ms Mueller relocated to the Turkish-Syrian border in December 2012 to help Syrian refugees, working with the Danish Refugee Council and the aid group Support to Life. She was taken by ISIS while leaving a hospital in the northern Syrian city of Aleppo in August 2013.
Her parents said they had previously remained silent about her capture "out of concern for Kayla's safety," and to abide by the group's warnings.
ISIS previously executed American journalists James Foley and Steven Sotloff and aid worker Peter Kassig, British aid workers David Haines and Alan Henning, Japanese journalist Kenji Goto and Mr Goto's friend Haruna Yukawa.
Among the hostages still thought to be held by the group is British photo journalist John Cantlie.
Ms Mueller was the last-known American hostage held by ISIS, which controls wide areas of Syria and Iraq.
The group has beheaded three other Americans, two Britons and two Japanese hostages - most of them aid workers or journalists - in recent months. Mueller was taken hostage while leaving a hospital in the northern Syrian city of Aleppo in August 2013.
The group's latest claim, detailed by the SITE monitoring group, came just days after it released a video on Tuesday showing captured Jordanian pilot Mouath al-Kasaesbeh being burnt alive in a cage.