Kayla Mueller, the American aid worker captured by the Islamic State in Iraq and Syria (ISIS), has died, the White House confirmed in a statement Tuesday.
"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," President Obama said in a statement.
The White House did not say how the 26-year-old humanitarian worker had been killed.
ISIS militants claimed last week that she had perished in a Jordanian airstrike against the group, but those reports remain unconfirmed. Photos discovered by the SITE terrorist monitoring group depicted an ISIS hideout destroyed in the Jordanian bombing, but did not show Mueller.
National Security Council spokeswoman Bernadette Meehan said that over the weekend, Mueller's family received a private message from her captors "containing additional information."
"Once this information was authenticated by the intelligence community, they concluded that Kayla was deceased," Meehan said.
CBS News reported that Mueller's family received an email from the terror group telling them their daughter had died, including a picture.
The group released videos depicting the deaths of other hostages, including the beheadings of three other Americans, two British citizens and two Japanese hostages. Last week, ISIS released a video showing a captured Jordanian pilot, Mouath al-Kasaesbeh, being burned alive in a cage.
Mueller was taken hostage in August 2013 while leaving a hospital in the Syrian city of Aleppo. She had worked with humanitarian organizations in India, Israel and the Palestinian territories, and assisted refugees in Turkey who had fled the Syrian civil war.
Mueller is thought to be the last known U.S. hostage held by ISIS.
“Kayla represents what is best about America, and expressed her deep pride in the freedoms that we Americans enjoy, and that so many others strive for around the world,” Obama said in the statement.
White House press secretary Josh Earnest said that the private message sent to the Mueller family did not include enough information for the intelligence community to determine a cause or time of death.
The death of Mueller is likely to intensify calls from Capitol Hill to ramp up military efforts against ISIS. Already, critics have charged that the White House should consider using ground forces in the battle against the terror network, which has captured large swaths of Iraq and Syria.
Presently, the U.S. and coalition partners’ military efforts against ISIS have been largely limited to airstrikes in support of Iraqi and Kurdish security forces, as well as Syrian rebel groups.
The White House is expected to unveil legislation sometime this week explicitly authorizing the use of military force against ISIS, although it is not clear whether that bill will prohibit the deployment of ground forces. The White House has repeatedly said it does not intend to deploy ground troops in the fight.
In an interview with NBC airing last week, Obama said the U.S. was “doing exactly what we should be doing” to fight the terror network.
Obama said redeploying troops to the region would prove ineffective without local buy-in.
"It is entirely possible for us to deploy 200 or 300,000 U.S. troops,” Obama said. “If we don’t have inside of Iraq or inside of Syria or inside of Afghanistan, both the capacity and will of people to fight for themselves, then any gains that are made eventually dissipate. So this takes longer, but it’s the right way to do things.”
—Kristina Wong and Ben Kamisar contributed. This story was updated at 1:00 p.m.