The beheading of British aid worker David Haines by ISIS has intensified fears for other Western hostages being held by the jihadist group.
Haines' grisly execution follows those of American journalists James Foley and Steven Sotloff in recent weeks -- all of them shown in videos posted online.
ISIS militants have killed thousands of Syrians and Iraqis in their violent onslaught aimed at building an Islamic caliphate across a wide area of territory.
But the beheading of Western captives carries greater shock value beyond the region -- and works as a powerful propaganda tool for a group that has drawn members from Western Europe and North America.
"ISIS looks at this as a low cost strategy," said Peter Neumann, professor of security studies at King's College London. "They are seeing that in order to capture the world's attention and recruit people, they no longer need to take down the World Trade Towers or hit the Pentagon."
As the United States builds an international coalition to take on the Sunni extremist group in both Iraq and Syria, uncertainty remains about exactly how many more Western hostages the militants have in their possession.
In the video of Haines' killing, the black-clad ISIS executioner places his hand on another hostage, whom he identifies as Alan Henning, a British citizen.
The implied threat is clear. Sotloff appeared in the same way in the video of Foley's beheading. Haines appeared similarly in the video of Sotloff's death.
On Sunday, Henning's family distributed an image of him holding a child at a refugee camp on the Syria-Turkey border.
The family asked media to use this image rather than the one of Henning in an orange jumpsuit kneeling beside his captor.
ISIS is believed to be holding a number of Americans, a U.S. official told CNN in August after Foley's execution. The official, who spoke on condition of anonymity, declined to identify them or say exactly how many Americans are being held.
CNN National Security Analyst Peter Bergen reports that ISIS is understood to be holding at least two other American citizens, as well as hostages from additional Western countries.
Two Italian women were reported in August to be among the militant group's newest captives. A Dane and a Japanese national were said to be seized along with the two Italians.
It's hard to know how many captives there are because governments, employers and families tend to keep kidnappings quiet for fear of putting the victims in greater danger while negotiators work to secure their release.
(CNN)
