The small Arizona town where Kayla Jean Mueller grew up began gathering in grief Tuesday upon learning that the 26-year-old aid worker who traveled the world on a quest to help others died while in the hands of Islamic State militants.

A small memorial on the courthouse plaza began to grow rapidly as word spread that Mueller's death had been confirmed.

The Islamic State group reported Friday that Muller, whose 18-month captivity had largely been kept secret in an effort to save her, had died in a recent Jordanian airstrike targeting the militants. On Tuesday, her parents and U.S. officials said they were now certain of her death, although officials said they could not confirm how she died.

"What a fine, fine woman and a tribute to Prescott," said 15-year resident Tina Nemeth. "It's just so sad, it really is, and everyone feels exactly the same. It's a shock it hit Prescott. We're not that big of a town."

The former territorial capital of Arizona has only recently begun to recover from a devastating 2013 wildfire that claimed the lives of 19 members of an elite firefighting squad. Stickers featuring the fire crew's logo and bearing the number "19" are still fixed to vehicles all around the town of 40,000 people.

The mountain town's picturesque courthouse lawn is still recognizable to some outsiders as the site of the dramatic martial-arts fight scene in the 1971 film "Billy Jack."

On Tuesday, it was filled with members of the media waiting to hear from Mueller's family, which lives about 10 minutes away at the end of a winding dirt road. Sheriff's deputies have blocked the road since Friday.

"We are heartbroken to share that we've received confirmation that Kayla Jean Mueller, has lost her life," Mueller's parents, Carl and Marsha Mueller, said in a statement released earlier. "Kayla was a compassionate and devoted humanitarian. She dedicated the whole of her young life to helping those in need of freedom, justice, and peace."

President Barack Obama said Mueller, who assisted humanitarian organizations working with Syrian refugees, "epitomized all that is good in our world."

"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," the president said.

The White House said Obama had spoken with Mueller's parents and offered his condolences and prayers.

Mueller is the fourth American to die while being held by Islamic State militants. Three other Americans — journalists James Foley and Steven Sotloff and aid worker Peter Kassig — were beheaded by the group.

Journalist Austin Tice disappeared in August 2012 while covering Syria's civil war. It's not clear what entity is holding him, but it is not believed to be the Islamic State group or the Syrian government, his family has said.

Mueller was taken hostage in August 2013 while leaving a hospital in Syria. Her identity was long kept secret out of fears for her safety.

Jordan, which has launched a barrage of strikes in recent days in retaliation for the gruesome killing of one of its pilots at the hands of the militants, disputed the group's report of Mueller's death.

In the U.S., there was growing certainty that the claim from IS about the airstrike was false. White House spokesman Josh Earnest said that Jordan's airstrikes had targeted an IS weapons compound near the group's stronghold of Raqqa in northern Syria that had been targeted before, and that there was no evidence of civilians in the area ahead of the strike.

Added a U.S. intelligence official: "She was not killed in that airstrike."

Mueller's parents released a letter Tuesday that their daughter had written them while in captivity. In the undated letter, Mueller said she was, "in a safe location, completely unharmed."

"I am also fighting from my side in the ways I am able + I have a lot of fight left inside of me," she wrote. "I am not breaking down + I will not give in no matter how long it takes."

Secretary of State John Kerry said the U.S. was "unshaken" in its resolve to defeat the Islamic State, a group he called an "ugly insult to the civilized world."

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Associated Press writers Ken Dilanian, Deb Reichmann and Julie Pace in Washington contributed to this story.