The U.S. government knows the names of a handful of American citizens who have linked up with radical Islamic fighters in Syria.
But sources say the actual number of U.S. recruits is likely in the dozens.
Beyond that, more than a thousand Western Europeans have joined the terrorists, around 300 from France and 500 from Great Britain, including the executioner who killed American journalist James Foley this week.
Most if not all of the Western fighters have passports that would allow them to travel freely to the U.S. The FBI is tracking the radicals who are known, but former counter-terrorism official Rick Nelson says it would only take one or two to slip through security.
"These terrorists that can come overseas and conduct individual style attacks, whether they are active shooter scenarios or explosive backpacks, those kind of things are the greatest threat to the United States," he said.
Intelligence sources say ISIS and other Syria-based terror cells do not have the capability to carry out a large-scale attack in the U.S. But given enough time and operating space, the terrorists could present a global threat - greater than that ever posed by Osama bin Laden.
"This is an organization that has an apocalyptic end-of-days strategic vision that will eventually have to be defeated," Joint Chiefs Chairman Gen. Martin Dempsey said.
Sources say there is no credible information pointing to an ISIS-related plot against the U.S. homeland. But, officials have incomplete intelligence and they are worried about what they don't know.