The father of American aid worker Kayla Mueller, who was killed while in the captivity of the Islamic State group, has accused the US government of prioritizing its policy of refusing to pay ransoms to terrorists over the lives of American citizens. In a preview of an interview set to air Monday on the US network NBC, Mueller's father, Carl, said he understood Washington's policy against negotiating with terrorists who demand ransoms for the release of kidnapped foreigners. However, he told NBC's 'TODAY' show that "on the other hand, any parents out there would understand that you would want anything and everything done to bring your child home. And we tried. And we asked. But they put policy in front of American citizens' lives." Mueller, a 26-year-old Arizona native, was confirmed to have died under circumstances that remain unclear about 18 months after she was abducted while leaving a hospital in northern Syria.Following the confirmation of Mueller's death on February 10, US President Barack Obama said the act of informing hostages' families that the United States would not pay ransom is "as tough as anything I do."He defended the policy, saying, "The reason is once we start doing that, not only are we financing their slaughter of innocent people and strengthening their organization, but we're actually making Americans even greater targets for future kidnappings."The circumstances of Mueller's death were unclear. Her family received an email and photograph earlier in February from her captors that enabled American intelligence to determine that she had been killed, US officials said.Islamic State said that Mueller was killed when Jordanian fighter jets bombed a building where she was being held outside Raqqa, a stronghold in Syria of the Islamist terrorist group. Jordanian and American officials have expressed doubt about Islamic State's account of her death.On February 11, a day after Mueller was reported dead, Obama sent Congress a request to authorize military force against Islamic State, saying the group "has committed despicable acts of violence and mass execution."