An international aid worker who was killed while held captive by the Islamic State was memorialized during a hometown service in Prescott, Ariz. by her father, who hailed her as a hero.

Carl Mueller, father of slain aid worker Kayla Mueller, read a letter at the memorial service that described how she discovered her life’s purpose was to help ease the suffering of others, coming close to breaking down in tears before regaining himself as he read the letter as Mueller’s brother rubbed his shoulder, according to an Associated Press report.

Mueller and her boyfriend, who is Syrian, were taken hostage in Syria. Her boyfriend was released, although he later came back to Syria to try to free her. They had been trying to help refugees there who had been struggling under the weight of constant warfare in their country.

On Feb. 10, her family and U.S. officials confirmed that she had died, with the militant group claiming it happened in a Jordanian airstrike but U.S. officials not saying one way or another.

She was captured in August 2013 leaving a Doctors Without Borders hospital.

A total of 500 people attended the memorial in a college auditorium in Prescott, the town where she grew up. She was hailed as the ultimate Good Samaritan. Sen. John McCain (R-Ariz.) also showed emotion during the service, his voice cracking during his tribute to her, where he said that Mueller had touched lives — even those of people she had never met before.

McCain said that even if retribution was delivered “swift and complete,” it wouldn’t make up for the fact that Kayla Mueller’s life was taking from her.

During the ceremony, a slideshow showed pictures from happier times in Mueller’s life, including a camping trip, nuzzling a horse, and sniffing flowers.

Her father arrived at the ceremony in a suit and tie but showed the crowd his socks, which were rainbow-patterned and he later revealed were taken from his daughter’s room.

Mueller had spent her years trying to raise awareness of HIV and AIDS. She also protested genocide in Darfur, Sudan, during her college days and traveled to Palestine, Israel, France, and India. In addition, she volunteered for work in a women’s shelter.