The Arizona aid worker who was killed while being held captive by ISIS in Syria was honored with a moving memorial service in her hometown on Saturday. Kayla Mueller, 26, was captured in August of 2013 after leaving a Doctors Without Borders hospital in Aleppo, Syria, and held for 18 months. It is believed she was killed by Jordanian airstrikes targeting ISIS in Raqqa last month. About 500 people, including Arizona Senator John McCain, attended a ceremony in an auditorium at Yavapai College in Prescott that was held to honor the memory of 'the ultimate Good Samaritan'. Scroll down for video . Kayla Mueller, 26, was honored with a memorial at Yavapai College in Prescott, Arizona, on Saturday . Her father, Carl Mueller (right), addressed  the service with some assistance from Kayla's brother Eric (left) Arizona Senator John McCain spoke to about 500 people at the ceremony . Speakers at the ceremony reminisced about Kayla's quick wit and an overflowing heart. Kayla's father, Carl Mueller, read a moving letter at the service, recounting his daughter's discovery that her life's purpose was to ease the suffering of others. Mr Mueller paused to regain his composure as he read the letter his daughter wrote while working an earlier job at an orphanage in India. As her father struggled to get through a particular passage, Kayla's brother Eric rubbed his shoulders. He quoted his daughter as saying: 'This is my life's work. But my family is my life.' Kayla's family members thanked the soldiers who risked their lives in a rescue attempt for her. They also tried to bring some levity to the ceremony and Kayla's father, dressed in a  dark suit and tie, made sure to lift up the legs of his pants to show off the rainbow socks he found in her room. Her parents have started a nonprofit organization called 'Kayla's Hands' designed to further her humanitarian efforts locally and internationally. Kayla was captured by ISIS in August of 2013 in Aleppo, Syria and held for 18 months before being killed . Kayla helped raise awareness of HIV and AIDS and worked the overnight shift at a women's shelter in Prescott . Sen. McCain's voice cracked with emotion as he delivered a short tribute to Mueller. He said 'I didn't have the privilege - the blessing - of knowing Kayla. I wish I had. 'I'd be a better man for knowing her. I'm sure of that. 'But her example of compassion and courage, her generosity, her unbowed humanity, have by her terrible sacrifice and her dignity in extremity reached so many people who did not know her as all of you knew her. 'We can try to give justice to her murderers' other victims and their families. 'But even if we succeed, and our retribution is swift and complete, we could not equal the rebuke that Kayla's life gave to the culture of death that robbed her of it. 'I never knew Kayla. That's my loss. But I won't forget her. That's my privilege and my responsibility.' Mr Mueller read a letter at the service revealing the discovery that Kayla's purpose was to ease suffering . Emotions ran high during the ceremony yesterday and Mr Mueller had to be comforted by his son (right) ISIS released images of a badly-damaged building in which they claimed Kayla had been killed by an airstrike . A slideshow at the ceremony flashed images of happy moments in Kayla's life. Childhood photos showed her on a camping trip, standing next to a snowman and laughing as a dog sat straight up in a chair at the table during a meal. Other images showed Kayla as an adult, affectionately nudging her face on a horse's neck in one photo and, in another frame, bending down to smell flowers. In her hometown, Kayla helped raise awareness of HIV and AIDS and volunteered for the overnight shift at a women's shelter. She protested genocide in Darfur, Sudan, while in college at Northern Arizona University in Flagstaff. She also traveled to the Palestinian territories, Israel, India and France. Sen. McCain said: 'We can try to give justice to her murderers' other victims and their families' The Arizona Diamondbacks wore a patch with 'KAYLA' on it during spring training games earlier this month . Sen. McCain's voice cracked with emotion as he delivered a short tribute to Mueller during the ceremony . Kayla and her Syrian boyfriend were both taken hostage. He was released, but he returned to Syria to try to free her. She became interested in Syria after her boyfriend told about the struggles of refugees there. Her death was confirmed Feb. 10 by her family and US officials. The Arizona Diamondbacks wore a black patch with 'KAYLA' on it during spring training games earlier this month. Team President Derrick Hall said the way the young humanitarian 'gave back to the world around her embodied many of the core values' of the Diamondbacks organization. He said Kayla 'will always have a place in the history of Arizona' and that the Diamondbacks are 'deeply saddened by her loss.' Kayla is the fourth American to die while being held by ISIS. Three others - two journalists and an aid worker - were beheaded. In each case, their captors demanded huge ransoms, which the United States has refused to pay, saying doing so would only encourage more kidnappings. President Barack Obama has defended that policy, although he has said it was extremely difficult to explain it to victims' families. Kayla and her mother Marsha Mueller (right). Her parents have started a nonprofit organization called 'Kayla's Hands' designed to further her humanitarian efforts locally and internationally . Kayla's family members - Eric, Marsha and Carl (left to right) are seen during the memorial service for her .
Kayla Mueller, 26, was captured by ISIS in August of 2013 in Aleppo, Syria . Was held for 18 months before being killed last month, likely by airstrike . International aid worker was honored in hometown of Prescott, Arizona . Emotional father said daughter's purpose was to ease suffering of others . 500 people attended Saturday service including Arizona Sen. John McCain .