Friends and family of Kayla Mueller gathered to honour her life in her hometown last night, just under two weeks after the aid worker was declared dead by Islamic State.

Speaking to the hundreds gathered in Prescott, Arizona, Mueller's brother, Eric, thanked the city for supporting the family, and read out a letter he'd written to his sister.

"You were my first friend, my best friend, my only sister. My heart aches for you and what you went through. I have a hole in my soul that will never be replaced. I miss you. I love you. I cannot believe you are gone from my life. I can feel you and your soul watching over me. I love you so much," he said.

"May God keep you from any more harm, any more hurt. You're in His hands now. You do not have to suffer any more. Only now will you be able to see how much you really did and truly did for this world by looking down on it from above."

Eric said his family would move forward, knowing that his sister is "still here in spirit".

"You can rest in peace knowing that you changed this world," he finished. "God works in mysterious ways – ways we don't agree with, ways we don't understand – but sometimes he takes his angels home early. I'll never stop loving you. I love you, Kayla."

The 26-year-old's death was confirmed by officials on February 10, four days after IS said she was killed in a Jordanian airstrike. She had been captured by militants in August 2013 while working in Syria.

At the candlelight memorial on Wednesday, Mueller's friends reportedly encouraged those attending to donate canned goods and money to charity, to continue her legacy of helping those most vulnerable. A live band opened the ceremony with 'He who began a good work in you', a song that Mueller's mother, Marsha, sang to her as a child.

"Everyone that knew Kayla seemed to think that would be something she would have liked to see – the community getting involved in service and giving," Ron Merrell, a pastor at Heights Church in Prescott who gave the introductory remarks, said ahead of the gathering.

He told AP that he was hopeful the ceremony would inspire hope for Mueller's family, and bring light into darkness – "something that Kayla Mueller was good at".

At one point, Mueller's father, Carl, lifted his candle towards the sky, a gesture that was copied by the rest of the crowd.

Last week, Mueller's aunts described her as a "free spirit".

"She had a quiet, calming presence. She was a free spirit, always standing up for those who were suffering and wanting to be their voice," Lori Lyon said of her niece.

Her parents have not spoken to the press, but said their daughter was a "compassionate and devoted humanitarian. She dedicated the whole of her young life to helping those in need of freedom, justice and peace," in a statement.

They also released the text of a letter Mueller wrote during captivity. In it, she said she had been treated with "the utmost respect [and] kindness," by her kidnappers.

"If you could say I have 'suffered' at all throughout this whole experience it is only in knowing how much suffering I have put you all through; I will never ask you to forgive me as I do not deserve forgiveness. 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," she wrote.

"I have been shown in darkness, light + have learned that even in prison, one can be free. 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... "

It concludes: "Please be patient, give your pain to God. I know you would want me to remain strong. That is exactly what I am doing. Do not fear for me, continue to pray as will I + by God's will we will be together soon."