New Details Emerge about Mueller's Time with ISIS

The world is mourning the death of Kayla Mueller, the 26-year-old American woman held captive by Islamic State. Washington confirmed her death Tuesday. She is the fourth American hostage killed by the Islamist group.

"Kayla has touched the heart of the world. The world grieves with us," Lori Lyon, Mueller's aunt, said.

As they try to accept her death, her family and friends remember the kind, brave, and spunky girl she was.

"Kayla was a compassionate and devoted humanitarian," her parents, Carl and Marsha Mueller, said in a statement. "She dedicated the whole of her young life to helping those in need of freedom, justice, and peace,"

The 26-year-old was captured by ISIS terrorists while she was working with war refugees in Syria. In a newly released letter to her parents she wrote that she was being treated with respect and kindness.

She also said, "By God + by your prayers, I have felt tenderly cradled in free-fall. I have been shown in darkness, light + have learned that even in prison, one can be free."

Family and friends are remembering 26-year-old Kayla Mueller who was killed while being held by Isis in Syria. Mueller had been held for a year and a half by the terrorist group. Those who knew her said she devoted her life to helping humanitarian causes. Watch more below.

ISIS claims Mueller was killed during recent Jordanian airstrikes. They sent her family photographic evidence over the weekend that she was dead.

U.S. officials say it hasn't been possible to determine when or how she actually died.

Some officials say they have reason to believe she may have been given over to an ISIS commander, possibly by forced marriage.

President Barack Obama said the United States did everything possible to save her, except pay a ransom.

For the first time on Tuesday, he admitted approving a risky rescue attempt to save her and other ISIS hostages, but U.S. forces were unable to locate them.

"It's as tough as anything I do, having conversations with parents who understandably want by any means necessary for their children to be safe," the president said.

Meanwhile, ISIS appears to be strengthening.

A new report claims that 20,000 foreign fighters from around the world are travelling to Syria to join the terror group.

As the ISIS threat grows, the U.S. government continues to debate the use of military force. Some critics say such indecisiveness may prove deadly.

"My concern is the president truly does not understand the nature of this danger," Former Virginia Gov. Jim Gilmore told CBN News.

"He doesn't understand that this is a guerilla war to try to defeat us by a thousand cuts, that they're going to undermine us piece by piece, little by little, until finally we can't do anything anymore," Gilmore said.

Experts warn it's only a matter of time before ISIS poses a direct threat to the United States.