Captive (illustration)
Captive (illustration)Thinkstock

Kayla Jean Mueller, who died as a hostage of Islamic State (ISIS) jihadists, took strength during captivity in her faith in God and the love of her family, she wrote in a letter relatives released on Tuesday.

"I have been shown in darkness, light + have learned that even in prison, one can be free," said a handwritten letter by Mueller which was smuggled out by fellow captives following their release by Islamic State jihadists.

The 26-year old humanitarian aid worker was driven by an unquenchable passion to help others, saying that service to others brought her closer to God.

In the letter released by her family after the White House confirmed her death, she said she managed to find glimmers of joy, even in captivity.

"I have come to see that there is good in every situation, sometimes we just have to look for it, Mueller wrote in the letter received by her relatives in early 2014.

"I pray each each day that if nothing else, you have felt a certain closeness + surrender to God as well + have formed a bond of love + support amongst one another," she wrote in the letter, secretly penned in snatches of time during her captivity.

Mueller described the place where she was being held as "a safe location," and said she was "completely unharmed + healthy (put on weight in fact)."

She also revealed times of doubt and darkness, but also resolve as she summoned the will to keep going.

"None of us could have known it would be this long but know I am also fighting from my side in the ways I am able + I have a lot of fight left inside of me," she wrote in her letter. "I am not breaking down + I will not give in no matter how long it takes."

The idealistic native of Prescott, Arizona, was captured by ISIS terrorists in August 2013, while leaving a Doctors Without Borders hospital in Aleppo, Syria.

Her humanitarian service over the years included volunteer work at a women's shelter in her hometown of Prescott, as well as work at an HIV/AIDS clinic.

At the time she was taken hostage, Mueller had been working along the Turkish-Syrian border, trying to help some of the thousands of refugees fleeing the Syrian civil war.

That conflict was but one of several where Mueller had hoped to make a difference.

Since her graduation from Northern Arizona University in 2009, she dedicated her life to helping those in need across the globe from India, to Israel, to the Palestinian Authority (PA) held territories.

Finding God in suffering

In a birthday letter to her father in 2011, Mueller tried to explain the reason for her drive to serve others so selflessly.

"Some people find God in church. Some people find God in nature. Some people find God in love; I find God in suffering," Mueller wrote, adding that her mission to relieve suffering of the less fortunate was her "life's work."

Mueller was the last remaining American hostage known to be held by the jihadist group, which has made a grisly ritual of killing a succession of hostages, then posting the brutal executions on the Internet.

Mueller's relatives only last week revealed that she was in fact being held by ISIS, after having gone to extraordinary lengths to  keep it secret during her one-and-a-half year captivity.

After ISIS last week announced that Mueller had died as a casualty of coalition bombing, her family continued to cling to the hope that she somehow had escaped death.

But over the weekend they received a "private message" from ISIS that allowed US intelligence services confirm her death, the White House and the family said.

The family said they are buoyed by the knowledge of how much good Mueller did during her short life.

"We are so proud of the person Kayla was and the work that she did while she was here with us. She lived with purpose, and we will work every day to honor her legacy," they said in a statement in which they also offered prayers "for a peaceful resolution of the conflict in Syria."

US President Barack Obama paid homage to Mueller's brief, purpose-driven existence.

"She epitomized all that is good in our world," the president said in a statement.

"She has been taken from us, but her legacy endures, inspiring all those who fight, each in their own way, for what is just and what is decent," Obama said.