The family of kidnapped U.S. aid worker Kayla Mueller has confirmed her death in Syria. Mueller’s captors, the Islamic State, had claimed Mueller was killed in a Jordanian airstrike last week. On Tuesday, the family said it had received proof she was killed, but it remains unclear how. Kayla Mueller’s aunt, Lori Lyon, paid tribute to her niece.

Lori Lyon: “She has done more in her incredible 26 years than many people can ever imagine doing in their lifetime. My daughter said to me, 'Things that were important to Kayla are finally getting the attention that they deserve.' Kayla has touched the heart of the world.”

Mueller moved to the Turkish-Syrian border in late 2012 to work with Syrian refugees. She had previously worked with refugees overseas including Tibetans in India, Africans in Israel, and Palestinians in the Occupied Territories. On Tuesday, President Obama said he is heartbroken by Mueller’s death, but defended U.S. policy blocking negotiations and ransom payments to militant groups like ISIS.