In a statement, her family said they had received information the 26-year-old was dead. Mueller, who was captured in August 2013, was a prisoner of the self-described Islamic State when she died.
White House spokesman Joshua Earnest would not say if the hostage was being held by the Islamic State, but he said that the U.S. was aware of at least one other American being held in Syria.
The extremist group ISIS is exploiting an informal finance network in Spain to pay its fighters, according to Spanish intelligence officials. The system is often used by immigrants to transfer money.
Repeated airstrikes on the self-described Islamic State are "the beginning of our retaliation" for the extremist group's brutal killing of a captured pilot, Jordan's foreign minister says.
The U.N. Committee on the Rights of the Child said it's reviewed reports of the mass execution of boys and of beheadings, crucifixions and the burying of children alive.
Jordan executed two prisoners in revenge for the horrific killing of the 26-year-old pilot, including the Iraqi woman who was sought by the so-called Islamic State for a hostage exchange.
Moderate Syrian rebels took up arms to topple President Bashar Assad. But the U.S. is planning to train them to fight against a different foe, the self-declared Islamic State, instead.