abroad - living and volunteering with aid groups in northern India, Israel and the Palestinian territories, then treated AIDS in Arizona

Grinning from ear to ear on the pages of her high school yearbook Kayla Mueller looks every bit the all-American teenager.

The lively girl from the small town of Prescott, Arizona is smart, gregarious and enjoys a wide circle of friends.

But on viewing these photos from her school years - obtained exclusively by Daily Mail Online - few would have guessed that years later this young woman would wind up in the hands of terror group ISIS.

Aid worker Mueller was taken captive by the Islamic State in Allepo, Syria, on August 4, 2013, when she was leaving a hospital.

Tragically, she is now the subject of grim world-wide headlines over fears the 26-year-old was killed during a Jordanian airstrike on the ISIS compound where she was held.

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Tragic: Kayla Jean Mueller, 26, (pictured in her year book photo) is understood to be the last U.S. hostage held by the terror group, which had been demanding $6.6m for her release. They claim she was killed last Friday

Popular: Kayla, pictured in her year book photo with the school robotics team, was voted Best Personality

Ambitions: School friends told Daily Mail Online that Kayla wanted to travel the world helping people

The US Government hasn’t been able to verify the claim and her parents refuse to accept it until they have official confirmation.

Some believe the claim is a cynical PR stunt concocted by ISIS commanders to cause friction between Jordan and the United States.

But others fear Mueller will never return home to Prescott, a picturesque community tucked away in the Bradshaw mountains, two hours north of Phoenix.

Daily Mail Online has been able to build a vivid picture of a young woman eager to help and serve.

At school Mueller is remembered as the girl who wanted to ‘change the world’ and it comes as no surprise to former classmates that she joined an international aid agency, travelling overseas to a far-flung places.

Friends at Tri-City College Prep high school in Prescott recall a ‘kind’, ‘big-hearted’ kid, who always did nice things for others.

High-flyer: At school in Prescott, Arizona, Kayla was part of numerous environmental and volunteering groups

Memories: Kayla's school friend Liz Peters, 25, (left) said Kayla (right) was enthusiastic about volunteering

As a high school student in Prescott, Mueller received awards for her volunteer efforts with groups such as AmeriCorps, America's Promise and Big Brothers Big Sisters.

She told the hometown newspaper, the Daily Courier, in 2007 that she was interested in world affairs and wanted to work with people in the strife-torn Darfur region of Sudan.

The young humanitarian even conducted two silent protest marches in her hometown to raise awareness surrounding the issues in Darfur in 2007.

She also wrote letters and placed calls to members of Congress to push for a change in U.S. policy.

School friend Liz Peters, 25, said: ‘I am not surprised about the path she chose, she always wanted to help people.

‘I remember that she wanted to change the world, community to community. She was hugely in to the environment.

‘We worked on the envirothon together, a state wide competition which our high school competed in.

‘We put together a project on the black-footed ferret being re-released in to the wild.

I remember that she wanted to change the world, community to community Kayla's childhood friend Liz Peters

‘Kayla loved science, her and the teacher, Dr David Somerville would go to Willow Lake and do water tests for pollution to make sure it was safe for the frog population.’

Peters also recalls Mueller being on the school robotics team, another venture run by her favorite teacher Dr Mueller.

‘We had a lots of disasters with a tennis ball cannon we were trying to build, but it was great fun,’ she recalled.

According to the high school yearbook the robotics team came ‘second only to Canada’ with the machine they sent to competition.

Mueller, who graduated in 2007, was also a member of the rowing team and the Science National Honor Society team.

At school she joined the ‘Leo Club’, a club which empowers volunteers to serve their communities and meets humanitarian needs.

In her graduating year Mueller was voted ‘Best Personality’, ‘Best Dressed’, and ‘Best Smile’ and several yearbook photos show her smiling with classmates.

Plea from parents: Marsha Mueller (right ) - whose daughter Kayla (left) is an ISIS prisoner - and her husband wanted to negotiate with their daughter's captors rather than risk her safety with a rescue mission

Shocking: American officials said they were looking into the reports if Kayla Jean Mueller's death. The White House said it was looking into the claim but that so far there was no evidence that corroborated it

In the same yearbook Mueller quotes William Blake as saying: ‘No bird soars too high if he soars with his own wings.’

The teen lived with her parents in a home accessible by only one road in a rural area north of Prescott called Williamson Valley.

Yesterday the road was closed off with Yavapai County sheriff's deputies standing by barricades and blocking the entrance to their street.

After high school Mueller attended Northern Arizona University, where she became big in to activism.

She travelled abroad, living and volunteering with aid groups in northern India, Israel and the Palestinian territories.

The common thread of Kayla's life has been her quiet leadership and strong desire to serve others Mueller family statement

She returned to Arizona in 2011, where she worked at an HIV/AIDS clinic and volunteered at a women's shelter.

At the clinic, she took patients vitals, and helped with fundraisers and community outreach.

Late that year, she moved to southeastern France and worked as an au pair while learning French in preparation for a planned move to Africa.

But the plight of families fleeing the violence in war-torn Syria drew Mueller to Turkey in December 2012.

She worked with the aid groups Support to Life and the Danish Refugee Council, assisting women and children who crossed into Turkey as refugees.

She also made some trips into Syria to help reconnect family members separated by the fighting.

‘The reason she is over there is, she wanted to help people,’ said Kennie Crotts, a close friend of Mueller's mother for 26 years.

‘I just can't even imagine the heartache,’ Crotts, who knew of Mueller's kidnapping. "I just knew she was going to be released."'

Speaking to the Prescott Daily Courier she said of Mueller's captors, ‘They just don't have compassion for anybody.’

Blast: ISIS released images of this badly damaged building in which they claimed Kayla Jean Mueller had been killed during an Jordanian airstrike

This graphic, shared on Twitter, claims to show the location of the building where ISIS allege Miss Mueller was killed during a raid by Jordanian forces

‘She was always there with a helping hand,' said Nancy Birck, a volunteer coordinator at Northland Cares HIV specialty care clinic, where Mueller volunteered, who described Mueller as ‘very compassionate and loving.’

‘She always had a smile on her face,’ she said.

When a Northland Cares worker was killed in a car accident, Mueller helped plan the memorial.

‘The common thread of Kayla's life has been her quiet leadership and strong desire to serve others,’ according to the family's statement.

She was recognized in her high school years as a National Young Leader and received the President's Award for Academic Excellence in 2007 and the Yavapai County Community Foundation Youth Philanthropist of the Year award in 2005.

Family and friends said they were shocked when her alleged death became headline news on Friday.

‘When I saw Kayla's face on the news ... Oh my gosh, she's a beautiful person inside and out,’ Birck told the Daily Courier.

She learned of Kayla's kidnapping about a year ago.

‘I just can't grasp it,’ she said.

‘For years, her parents have been wondering what happened to her.

‘I can't imagine as a mom what she (Marsha) is going through.’

Her school friend Peters, now a teacher at Yavapai College, added: ‘What has happened to Kayla is awful news.

'I'm sure everyone will be hoping it’s not true and she can come home.’