US President Barack Obama confirmed on Tuesday that Kayla Mueller, an American aid worker held hostage by the Islamic State group, was dead, adding that those responsible would be found and brought to justice.

"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," he said in a statement.

Mueller’s family also said in a statement that they were “heartbroken” to learn of her death and released a copy of a letter she had written in 2014 while in captivity.

The Islamic State (IS) group, also known as ISIS or ISIL, claimed last week that she had been killed in a bombing by Jordanian fighter jets.

The militant Islamist group, which controls wide areas of Syria and Iraq, has beheaded three other Americans, two Britons and two Japanese hostages – most of them aid workers or journalists – in recent months.

Originally from the small city of Prescott about 100 miles (160 km) north of Phoenix, Mueller’s family described her in a recent statement as someone who felt compelled to help others from an early age.

As a high school student at Tri City College Prep, she received several awards, in part for her volunteering with groups like AmeriCorps and Big Brothers Big Sisters, the statement said.

She graduated from Northern Arizona University in 2009 and went on to work for humanitarian aid groups in northern India, Palestine, and Israel before returning to Arizona to work at an HIV/AIDS clinic and volunteer at a woman’s shelter, it said.

Mueller relocated to the Turkish-Syrian border in December 2012 to help Syrian refugees, working with the Danish Refugee Council and the aid group Support to Life. She was taken by the IS group while leaving a hospital in the northern Syrian city of Aleppo in August 2013.

(FRANCE 24 with AFP, REUTERS)