(CNN) -

With the beheading of a second American journalist, Steven Sotloff, militant group ISIS delivered another brutal message to the Western world.

And, as with the execution of journalist James Foley before him, ISIS ended its video -- titled "A second message to America" -- with a threat to another Western hostage, this time a Briton.

The three are not the only Westerners held by the militant group. And their plight has raised fears for the safety of all those taken hostage.

Thousands of Syrians and Iraqis have been slain by ISIS militants as the Sunni extremist group seeks to build an Islamic caliphate stretching across a swath of territory.

But the execution of Western captives holds greater shock value outside the region's borders -- and represents a powerful propaganda tool.

CNN terrorism analyst Paul Cruickshank said that the method of killing has a specific purpose for ISIS.

A video like the one showing Sotloff's killing "really energizes" supporters of ISIS, and beheading is employed for "maximum propaganda" to "terrify" ISIS' enemies, Cruickshank said.

Alternatively, Western hostages, often journalists or aid workers, can be a useful tool if kept alive: either freed for hefty ransoms, used as bargaining chips for the militants' ends or sold on to other extremist groups.

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.

ISIS is believed to be holding a number of Americans, a U.S. official told CNN 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.

Missing 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.

Among them is American Austin Tice, a freelance journalist who was contributing articles to The Washington Post. He disappeared in Syria in August 2012. There has been no word of or from him since his abduction.

Altogether, more than 80 journalists have been abducted in Syria since the conflict started, according to the CPJ.

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

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

It emerged after Foley's death that U.S. special operations units were sent into Syria this summer to rescue him and other hostages, but their mission proved unsuccessful.

Richard Byrne, a spokesman for the news website GlobalPost, also revealed that ISIS had demanded a ransom of 100 million euros (about $132 million) for Foley's release. The journalist freelanced for GlobalPost and other news organizations.

After his death, Foley's mother, Diane, expressed concern about other hostages on the Free James Foley Facebook page.

"We implore the kidnappers to spare the lives of the remaining hostages. Like Jim, they are innocents. They have no control over American government policy in Iraq, Syria or anywhere in the world," she wrote.

Sotloff's mother, Shirley Sotloff, also had directed an emotional plea for her son's release to ISIS leader Abu Bakr al-Baghdadi, to no avail.

To pay or not to pay?

Fears for the safety of Westerners held by Islamist militants have also highlighted the different approaches taken by governments when it comes to kidnappings.

The UK government has a hard-line policy of never paying ransoms. The U.S. government also does not negotiate with terrorists. At the same time, other European governments in the past are thought to have handed over cash to terror groups to ensure the release of nationals.

The United States, as in the case with the thwarted Foley mission, has in the past attempted hostage rescues.