Support for the Islamic State is growing from groups outside the Middle East as a large number of Western Muslims reportedly have joined ISIS in Syria and Iraq.
What does this mean for the United States and other Western countries?
The ISIS terrorist shown beheading American journalist James Foley speaks English with a British accent.
That fact raised so much concern in the United Kingdom, Prime Minister David Cameron cut his vacation short to address the issue.
"From what we've seen it looks increasingly likely that is a British citizen. Now this is deeply shocking," Cameron said.
How likely is it that ISIS will strike America at home? CBN News Terrorism Analysist explains this and more below.
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British authorities believe several hundred of their citizens could be fighting with ISIS. But that's just a fraction.
Some estimates put the number of Western Muslims joining ISIS in Syria and Iraq up to 3,000.
In addition to Britain, they come from France, Denmark, and the United States, creating a serious danger for the West.
The concern is these jihadists will use their Western passports and terrorist training to return to their country and carry out attacks at home.
"Some of those foreign nationals... may at some stage seek to return to their countries of origin and carry out attacks in them," British Foreign Secretary Philip Hammond said.
"We will be vigilant and we will be relentless. When people harm Americans, anywhere, we do what's necessary to see that justice is done," President Barack Obama said Wednesday. "There has to be a common effort to extract this cancer so that it does not spread."
Yet CBN News Terrorism Analyst Erick Stakelbeck said the president is not doing enough to eliminate the threat to America.
"Right now I'm not seeing the U.S. government under the Obama administration taking the kind of action needed to crush ISIS," Stakelbeck said. "We need to crush this group -- stamp them out before they pose a direct threat to the U.S. mainland."
Other Islamic terrorist groups, not just from the West, are now supporting ISIS, including from the Philippines and Yemen.
The Obama administration announced Wednesday that the president sent Special Ops forces to Syria this summer on a secret mission to rescue American hostages, including Foley, but they did not find them.
Obama has stepped up airstrikes against ISIS, but to wipe out that terrorist group, a lot more military action may be required.