(Adds Obama comments on ransom, Pentagon comment, paragraphs 20-22)
By Jeff Mason and Roberta Rampton
WASHINGTON, Feb 10 (Reuters) - U.S. aid worker Kayla Mueller, held hostage by Islamic State militants for 18 months, is dead, her family said on Tuesday, but the circumstances were unclear and President Barack Obama vowed to hunt down the culprits.
Mueller's family received an email and photograph over the weekend from her captors that enabled American intelligence to determine that she had been killed, U.S. officials said.
Islamic State said on Friday that Mueller, 26, was killed when Jordanian fighter jets bombed a building where she was being held outside Raqqa, a stronghold in Syria of the Islamist militant group. Jordan and U.S. officials have expressed doubt about Islamic State's account of her death.
Mueller, from Prescott, Arizona, was thought to have been the last American held hostage by Islamic State, an al Qaeda offshoot that controls parts of Syria and Iraq. But on Tuesday, White House spokesman Josh Earnest said, "We are aware of other American hostages being held in the region." He did not identify them or provide details.
Among hostages thought to be held by Islamic State is British photojournalist John Cantlie, captured in northern Syria in November 2012.
American journalist Austin Tice disappeared in Damascus in August 2012, and last week his family renewed its campaign to draw attention to his case. It is unclear who is holding Tice or if he is still alive.
Mueller's family said in a statement they were "heartbroken" to learn of her death, and released a letter she wrote in 2014 while in captivity.
She was captured in August 2013 while leaving a hospital in Aleppo in northern Syrian. Mueller had previously worked in Turkey providing humanitarian assistance to refugees from Syria's civil war.
In recent months, Islamic State has beheaded three Americans, two Britons, and two Japanese hostages, most of whom were aid workers or journalists.
U.S. SEEKS JUSTICE
"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," Obama said in a statement issued by the White House.
"ISIL is a hateful and abhorrent terrorist group whose actions stand in stark contrast to the spirit of people like Kayla," Obama said, using an acronym for the group.
The White House said Obama called Mueller's parents to offer condolences.
Last year, Obama ordered air strikes on Islamic State positions, and the United States is leading an international coalition against the group in Iraq and Syria.
U.S. officials said they had no evidence to support Islamic State claims Mueller was killed in a Jordanian air strike.
Two U.S. national security officials who have closely followed Mueller's situation said it appeared most likely she was killed in some kind of combat situation in which her captors were unable to keep her safe.
Even after Islamic State announced Mueller's death on Friday, her family expressed hope that she was still alive.
The family released a handwritten letter they said Mueller wrote to them last spring. In it, she stated she was "in a safe location, completely unharmed + healthy" and had been treated with "the utmost respect + kindness." She wrote "just the thought of you all sends me into a fit of tears."
"I will never ask you to forgive me as I do not deserve forgiveness," she wrote.
"I have been shown in darkness, light + have learned that even in prison, one can be free."
Asked about Mueller's family soliciting money to pay ransom to gain her release, the White House's Earnest reiterated that it is U.S. government policy not to pay such ransom because the money would be used to fund terrorist groups.
In an interview on Tuesday, Obama told BuzzFeed News that telling hostages' families the United States would not pay ransom is "as tough as anything I do." He defended the policy, saying, "The reason is once we start doing that, not only are we financing their slaughter of innocent people and strengthening their organization, but we're actually making Americans even greater targets for future kidnappings."
Pentagon spokesman Rear Admiral John Kirby dismissed Islamic State's assertion that the Jordanian air strike was responsible for Mueller's death. "Let's not forget in whose hands this woman died. And let's not forget who's ultimately responsible for it: ISIL," Kirby said.
Mueller went to Turkey in December 2012 to work for a Turkish organization providing humanitarian aid to Syrian refugees along the border with Syria. She previously volunteered for schools and aid groups abroad including in the West Bank, Israel and India.
Mueller's family quoted from another letter she sent her father on his birthday in 2011: "I will always seek God. Some people find God in church. Some people find God in nature. Some people find God in love; I find God in suffering. I've known for some time what my life's work is, using my hands as tools to relieve suffering." (Additional Reporting by Jeff Mason, Phil Stewart, Mark Hosenball, Alistair Bell, Emily Stephenson, Julia Edwards and Roberta Rampton; Writing by Will Dunham; Editing by Susan Heavey, Toni Reinhold)