A sign for Kayla Mueller is displayed along a main street in Prescott, Arizona, Feb. 10, 2015. Mueller was the last-known American hostage held by Islamic State group militants. U.S. officials said the family received an email and photo from her captors that confirmed her death. Reuters

An American aid worker killed in February while held hostage by Islamic State group militants in Syria was raped by the group's leader, ABC News reported Friday. Kayla Mueller, 26, was repeatedly raped by Abu Bakr al-Baghdadi, officials said. Her family was informed of the abuse in June, the BBC reported.

Mueller was abducted in 2013 while working in Aleppo, Syria. "We were told Kayla was tortured, that she was the property of Baghdadi. We were told that in June by the government," her parents, Carl and Marsha Mueller, told ABC News.

Before Mueller's death, Baghdadi reportedly took the aid worker as a "wife." He often visited the compound where she was being held and repeatedly assaulted her. At least two teenage Yazidi girls who were held hostage as sex slaves confirmed the attacks.

“I saw him often, he came there often,” one Yazidi girl said. “And every time he came, he took Kayla away. She would always come back after.”

Officials had previously said Mueller was treated well in captivity because of a letter she wrote in 2014 that was smuggled out to her family. In it, Mueller tried to ease her family's concerns, saying she had been treated with "utmost respect + kindness."

Mueller, of Prescott, Arizona, traveled to the Turkey-Syria border in 2012 to work with refugees. She would have turned 27 on Friday.

Mueller reportedly learned basic Arabic while in captivity and went by the name of “Kayla Carol." The last name referenced her father’s name, Carl, as is Arab practice, the Independent reported.

Mueller was kidnapped in Syria while leaving a Spanish Médecins Sans Frontières [Doctors Without Borders] hospital in Aleppo. The Islamic State group -- also known as ISIS or ISIL -- demanded a ransom of 5 million euros ($5.5 million) for her release.

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Announcing her death in February, President Barack Obama said: “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.” Mueller, he said, had “epitomized all that is good in our world."

Her parents said they were heartbroken by her death. “Kayla dedicated the whole of her young life to helping those in need of freedom, justice and peace," they said in a statement at the time.