American Aid Worker Kayla Mueller Killed at Hands of ISIS 'Surrendered to God' During Captivity, Drew Comfort From 'Deep Christian Faith'

(Photo: REUTERS/Courtesy Mueller family)Kayla Mueller in an undated photo.

The United States confirmed on Tuesday that 26-year-old American aid worker Kayla Mueller has been killed at the hands of terror group ISIS. President Barack Obama said that Mueller represented what is "best about America," while her family revealed that she drew comfort from her "deep Christian faith" while in captivity.

"Our hearts are breaking for our only daughter, but we will continue on in peace, dignity, and love for her," Mueller's parents, Carl and Marsha Mueller, and her brother, Eric, said in a statement.

The aid worker had been held hostage by ISIS since August 2013, The Guardian noted. Supporters of ISIS said last week that Mueller had been killed during a Jordanian air strike, and though the U.S. confirmed the news, it could not determine the cause of her death.

Mueller was reportedly taken by militants in Syria while working at a hospital run by humanitarian group Médecins Sans Frontières. In a letter to her family written sometime in the spring of 2014, she asked them not to worry and claimed that she was being treated with respect and was unharmed.

She said that her deep Christian faith gave her comfort during her captivity:

"I remember mom always telling me that all in all in the end the only one you really have is God. I have come to a place in experience where, in every sense of the word, I have surrendered myself to our creator b/c literally there was no else ... + by God + by your prayers I have felt tenderly cradled in freefall."

The aid worker had travelled to Syria to help provide aid to refugees and those suffering in the ongoing civil war. Before that she also worked for humanitarian initiatives in India, Israel and the Palestinian territories.

Mueller said in a 2013 interview with the Prescott Daily Courier, a newspaper from her hometown of Prescott, Arizona: "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."

Fox News revealed further details about Mueller's capture in a report on Wednesday, and noted that she had spent several months being transferring between holding cells — and was held both at a children's hospital in Aleppo, and the ISIS stronghold of Raqqa.

Obama revealed that the U.S. had made an attempt to rescue Mueller, but the operation was unsuccessful.

"I deployed an entire operation — at significant risk — to rescue not only her, but the other individuals who had been held, and probably missed them by a day or two," Obama told Buzzfeed in an interview.

Fox News' Catherine Herridge reported Tuesday that the Obama administration knew the location and building where Mueller and the other Western hostages were being held but waited more than five weeks before attempting a rescue.

Mueller became the fourth U.S. citizen to die at the hands of ISIS, following fellow aid worker Peter Kassig and journalists James Foley and Steven Sotloff in 2014. The president said, however, that no matter the cost, paying ransom to the jihadists is not an option.

"It's as tough as anything that I do — having conversations with parents who understandably want, by any means necessary, for their children to be safe," Obama said. "We will do everything we can short of providing an incentive for future Americans to be caught."

Obama vowed to bring those responsible for Mueller's death to justice.

"Kayla represents what is best about America, and expressed her deep pride in the freedoms that we Americans enjoy, and that so many others strive for around the world," he added in a White House statement. "In how she lived her life, she epitomized all that is good in our world."

Mueller's family said that they are proud of her work, and noted that she lived with a purpose.

The family added: "We remain heartbroken, also, for the families of the other captives who did not make it home safely and who remain in our thoughts and prayers. We pray for a peaceful resolution of the conflict in Syria."