Kayla Mueller in an undated photo provided by the Mueller family. The 26-year-old aid worker from Arizona was kidnapped in August 2013
Since August 2014, the Islamic State extremist group has publicized the deaths of westerners and others fighting their occupation of Iraq and Syria.
Kayla Mueller in an undated photo provided by the Mueller family. The 26-year-old aid worker from Arizona was kidnapped in August 2013.
ISIS claimed she died during a Jordanian air strike in February 2015
Jordanian pilot Muath al-Kasaesbeh crashed while flying a mission over Syria and was executed soon afterward.
A video spread by group on Feb. 3, 2015 shows Kasaesbeh being burning a live in a cage.
In this still image from an Oct. 3, 2014 video released by Independent Press and Pacific Bridge Media & Consulting, Japanese journalist Kenji Goto reports from a Kurdish town on the outskirts of Kobani, Syria.
The Islamic State group released a video on Tuesday, Jan. 20, 2015, threatening to kill two Japanese hostages, Goto, 47, and Haruna Yukawa, 42, within 72 hours unless it receives $200 million.
An image from video posted to Haruna Yukawa's blog shows him receiving weapons training with Syrian rebels in early 2014.
Haruna Yukawa was captured in August 2014 and executed by ISIS militants in January 2015.
After a number of setbacks at home, Yukawa set himself up as a private military contractor despite having no prior military experience.
British journalist John Cantile, shown here, has appeared in several pro-ISIS propaganda videos. One, released in January 2015, shows Cantile giving a tour of mosul, Iraq. Another, he reports from Kobani.
Herve Gourdel was kidnapped on September 21, 2014, while trekking in the rugged, heavily forested Kabylie area of Algeria, where al Qaeda is active. He was beheaded three days later.
The sign that reads "Herve Gourdel, come back," was placed in front of the town hall of Saint-Martin-Vesubie, southeastern France.
Peter Kassig, the American founder of the relief organization Special Emergency Response and Assistance (SERA) and an Iraq War veteran, in an undated photo.
Kassig was captured in October 2013 and kept with western hostages Nicolas Henin and John Cantile.
A video released on November 16, 2014 shows his beheading.
Alan Henning, a 47-year-old British aid worker, was providing medical aid for injured refugees in Syria when he was captured near the Syrian town of Al Dana in December 2013.
A video released in October 2014 depicts his beheading.
David Haines and his wife, Dragana Haines. Haines, a British aid worker, was assisting refugees along the Turkish border when he was kidnapped in August 2013.
U.S. journalist Steven Sotloff is pictured in this undated handout photo.
Islamic State insurgents released a video on August 19, 2014 showing the beheading of U.S. journalist James Foley and images of Sotloff whose life they said depended on U.S. action in Iraq.
Steven Sotloff, center, talks to Libyan rebels on the Al Dafniya front line, west of Misrata on June 2, 2011 in Misrata, Libya.
Sotloff was kidnapped in August 2013 near Aleppo, Syria and was recently shown on a jihadist video in which fellow US journalist James Foley was executed. His beheading was shown in a video released by ISIS on September 2, 2015.
James Foley,a freelance journalist from New Hampshire, is seen in Benghazi, Libya, in a still from a 2011 video.
Foley was the first westerner publicly killed by ISIS in August, 2014.
This November 2012 file photo, posted on the website freejamesfoley.org, shows American journalist James Foley while covering the civil war in Aleppo, Syria.
Foley was captured in 2012 and a video showing his execution was released in August 2014.