"Kayla was a compassionate and devoted humanitarian. She dedicated the whole of her young life to helping those in need of freedom, justice and peace," her parents said.
Mueller's Christian faith is now in the spotlight, as her parents share a personal letter they received from Mueller's fellow hostages. The letter, published by multiple news sources, reveals how Mueller's faith held her during her captivity.
"I have been shown in darkness, light + have learned that even in prison, one can be free," Mueller wrote. "I am grateful. I have come to see that there is good in every situation, sometimes we just have to look for it. 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 also wrote how her parents' faith influenced her as well.
"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."
Despite her death, Mueller instructs her family and friends how to handle their grief.
"Please be patient, give your pain to God. I know you would want me to remain strong. ... by God's will we will be together soon."
Mueller was captured by ISIS in Syria in August 2013, and was the only known remaining U.S. hostage held by the Islamic State group.
U.S. President Barack Obama says the U.S. is seeking out Mueller's killers.
"No matter how long it takes, the United States will find and bring to justice the terrorists who are responsible for Kayla's captivity and death," he says.
For the full letter, click here.