Kayla Mueller, the US hostage whose death was confirmed yesterday, may have been given to an Isis commander, according to reports from officials.

A week after Isis militants claimed that the aid worker had died in a Jordanian airstrike, the 26-year-old’s family, who live in Prescott, Arizona, confirmed she had lost her life, although it was not immediately clear when or how.

The Pentagon has said it does not know how she died, but said it is certain it was not during the airstrike.

Ms Mueller is the fourth American to die while being held by Isis militants journalists James Foley and Steven Sotloff, and aid worker Peter Kassi, were beheaded by the group.

Over the weekend, Isis sent the family an email which included a photograph of Ms Mueller’s body that indicated she had died “recently”, Bernadette Meehan, a spokesman for the Muellers said yesterday.

Today, more information surrounding Ms Mueller’s treatment in captivity has emerged, as a counter-terrorism official told ABC News that Isis didn’t regard Ms Mueller as a “hostage or a bargaining chip” like other captives.

As the US collects intelligence in Syria using satellites and drones rather than human sources and spies, the limited information compiled by officials appeared to show Isis fighters referring to Ms Mueller in calls, or guarding a sensitive area where she may have been held, an official told ABC.

Intelligence also indicated that Ms Mueller was sometimes in the company of an Isis leader who had custody over her – possibly by forced marriage.

Matt Olsen, former head of the National Counterterrorism Centre and current ABC News consultant, said ISIS is known for the “practice of essentially selling off, or putting young women and girls in the company of ISIS militants."

Ms Mueller was kidnapped in August 2013, but her captivity was kept secret in an effort to secure her freedom.

In the wake of her death, President Barack Obama confirmed that a military operation last summer to recover Mueller and others hostages failed when rescuers arrived only ”a day or two“ after the group had been moved.

Mr Obama pledged to bring Mueller's captors to justice "no matter how long it takes." The White House said Mr Obama had spoken with Mueller's parents and offered his prayers.

On Tuesday, her family spoke fondly of Ms Mueller's free spirit, and her efforts to ease the suffering of others - which stretched beyond Arizona to the Palestinian territories, Israel, India, France and Syria.

Her family encouraged people to donate to organisations the aid worker would have supported, arguing that big displays of support would be against her humble nature.

"I'm not sure yet how to live in a world without Kayla, but I do know that we're all living in a better world because of her," said a tearful Eryn Street, one of Mueller's closest friends, as she spoke from the courthouse plaza in Prescott on Tuesday.