Kayla Mueller, 26, an American humanitarian worker from Prescott, Arizona is pictured in this undated handout photo obtained by Reuters February 6, 2015. US President Barack Obama on February 10, 2015 confirmed the death of Mueller, a US aid worker who had been held hostage by Islamic State militants. Kayla Mueller was abducted in Aleppo, Syria in August 2013. [Photo/Agencies]

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

A memorial of flowers and handwritten notes took shape on the courthouse plaza in Prescott near a sign calling on people to pray for her.

In Washington, President Barack Obama pledged to bring Mueller's captors to justice "no matter how long it takes."

Muller's 18-month captivity had largely been kept secret in an effort to save her. The Islamic State group claimed Friday that she had died in a recent Jordanian airstrike targeting the militants.

On Tuesday, her parents and US officials confirmed her death. The Pentagon said US officials don't know how or when she died but are certain it was not in the Jordanian airstrike.

"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 town.

The mountain town of 40,000 people resembles a relic of the Old West in many ways, with its colorful downtown saloons and a dirt road leading out of town to where Mueller's family lives. Its picturesque downtown courthouse lawn is recognizable to outsiders who still recall it as the site of the dramatic martial-arts fight scene in the 1971 film "Billy Jack."

On Tuesday, that lawn was crammed with members of the media gathered to hear an emotional, often tearful tribute from Mueller's family and friends.

"All these stories about Kayla, she sounds so extraordinary,"said the Rev. Kathleen Day, who heads the United Christian Ministry at Northern Arizona University, where Mueller attended college.

"What was so extraordinary about Kayla was she did ordinary things to extraordinary measures,"Day continued. "She gave people food. She gave people water."She even befriended her captors, the reverend added, at one point trying to teach them origami.

And she wrote passionately about conditions in war-torn Syria, where she had gone to help refugees.

"Every human being should act. They should stop this violence,"Day said, quoting one of Mueller's blog posts.

Her aunt Lori Lyon said Mueller accomplished more in her 26 years than most people do in a lifetime, adding that her death had "touched the heart of the world."

From Jordan, government spokesman Mohammed al-Momani offered his country's condolences.

The White House said Obama had spoken with Mueller's parents and offered his prayers. The president said Mueller "epitomized all that is good in our world."

Arizona Sen. John McCain hailed Mueller's humanitarian work in a speech from the Senate floor.

"After graduating from Northern Arizona University in Flagstaff in 2009, Kayla committed her life to helping people in need around the world _ first in India, then Israel and the Palestinian territories and back home in Prescott, where she volunteered at an HIV/AIDS clinic and a women's shelter,"he said.

As a high school student in Prescott, McCain noted, Mueller was recognized as a leader and received the President's Award for Academic Excellence, as well as other honors.

Mueller is the fourth American to die while being held by Islamic State militants. Three others _ 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.

In each case, their captors demanded huge ransoms, which the United States has refused to pay, saying doing so would only encourage more kidnappings. Obama defended that policy Tuesday in an interview with BuzzFeed News, although he said explaining it to victims' families is "as tough as anything I do."

He also said a military operation last summer to recover Mueller and others failed when rescuers arrived only "a day or two"after the group had been moved.

Islamic State said last week that Mueller was killed in a recent airstrike Jordan launched as retaliation for the militant group's gruesome killing of one of its pilots, who was burned to death.

Jordan denied that, and on Tuesday a Pentagon spokesman, Navy Rear Adm. John Kirby, said there was "no doubt"Islamic State killed Mueller. He said officials have not learned yet how she died.

Her parents released a letter 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."