Story highlights ISIS is believed to be have at least two more U.S. captives

It is also holding hostages from other Western countries

The latest beheading video identified a British citizen

It's not known who's holding U.S. journalist Austin Tice, who disappeared in 2012

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.

Has ISIS identified any of its other Western captives?

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.

Are any other Americans being held?

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.

Have the identities of any missing Americans been made public?

The family of one missing U.S. journalist, Austin Tice, has talked to the media about his case. But it's not clear which group in Syria's brutal civil war might be holding him.

Tice, a freelance journalist who was contributing articles to The Washington Post and other news outlets, disappeared in Syria in August 2012.

A shaky video uploaded to YouTube in October 2012 showed Tice blindfolded and in obvious distress. It was unclear who the people holding him were.

The U.S. State Department said at the time that it believed Tice was in the custody of the Syrian government -- but the regime of President Bashar al-Assad hasn't admitted detaining him.

Tice's parents spoke last week to CBS This Morning about their concerns for their 33-year-old son.

"I do not accept that he is missing, I live in a place where he is coming home," said Debra Tice, his mother. She described hearing the news of the killings of Foley and Sotloff as "a gut punch."

What about other journalists?

The Committee to Protect Journalists estimates that about 20 journalists are missing in Syria -- most of them local, some from outside Syria. It says many of them are believed to be held by ISIS.

Altogether, more than 80 journalists have been abducted in Syria since the country's civil war started, according to the CPJ.

Some have been freed, others killed. Often a media blackout on journalists' abductions is lifted generally in best- and worst-case scenarios.

Which Westerners have been released in Syria?

American journalist Peter Theo Curtis was handed over to U.N. peacekeepers in August by al-Nusra Front, a Syrian rebel group with al Qaeda ties, after almost two years in captivity. Qatari officials helped secure his release, his family said.

Two Spanish journalists -- Javier Espinosa and Ricardo Garcia Vilanova -- were freed in March. At the time of their capture, their newspaper said the journalists' captors were reported to be members of a group linked to ISIS.

Four French journalists -- Nicolas Henin, Pierre Torres, Didier Francois and Edouard Elias -- were released in April. And a Danish photographer, Daniel Rye Ottosen, was freed in June, the Danish Foreign Ministry said.

Why are some hostages freed and others killed?

A multitude of factors are at work, including who the captors are and where the captives come from.

Different Western governments take different approaches to kidnappings.

Britain and the United States are known to take hard line, refusing to negotiate with terrorists or pay ransoms. But some European governments are believed to have handed over cash to terrorist groups to ensure the release of their citizens.

An investigation by The New York Times found that al Qaeda and its affiliates have netted at least $125 million in ransoms since 2008, with much of the revenue reportedly coming from France -- although the French government denied paying money to terrorists.

What about getting the hostages out by force?

The United States and other countries attempted hostage rescues, but these have achieved mixed results.

In 2012, Navy SEALs saved U.S. aid worker Jessica Buchanan and fellow aid worker Poul Thisted of Denmark after three months' captivity in Somalia.

But the attempts can also go badly wrong. Linda Norgrove, a kidnapped British aid worker, was accidentally killed by a U.S. grenade during a SEAL rescue effort in Afghanistan in 2010.

U.S. special operations forces went into Syria in July to try to retrieve Foley and other ISIS captives. But the mission failed after the commandos were unable to find the hostages.

The British government said earlier this month an attempt was made to rescue Haines "some time ago" but was unsuccessful. It declined to provide details on what happened.