But he's far from alone.
"Some of the foreign fighters may not return as terrorists to their respective countries, but all of them will have been exposed to an environment of sustained radicalization and violence with unknowable but worrying consequences," Richard Barrett of The Soufan Group wrote in a report called "Foreign fighters in Syria" this summer.
So what is it about ISIS and its uncanny ability to recruit Westerners? Here are five methods the group employs:
It preys on a recruit's sense of identity
The recruits are often young -- sometimes disillusioned teenagers trying to find purpose and make their mark.
For many, it boils down to a lack of a sense of identity or belonging, Barrett said.
"The general picture provided by foreign fighters of their lives in Syria suggests camaraderie, good morale and purposeful activity, all mixed in with a sense of understated heroism, designed to attract their friends as well as to boost their own self-esteem," he wrote
And some are drawn by the desire to die as a martyr who would enjoy a sumptuous afterlife if they die fighting the "infidel" enemy.
It appeals to their religious duty
Westerners waging jihad, fighting alongside ISIS and other terror groups, is not a new phenomenon. The FBI and the U.S. Justice Department have been trying to stay ahead of wannabe terrorists since the 9/11 terror attacks.
But what draws Americans to give up their comfortable lives in the United States and travel to war-torn countries to fight? An appeal to their sense of duty.
It's a message frequently posted by ISIS on social media: "You have to join. It's your religious duty," CNN terrorism analyst Paul Cruickshank said.
It operates a sophisticated propaganda machine
ISIS now operates the most sophisticate propaganda machine of any terrorist organization, said Matthew Olsen, director of the National Counterterrorism Center.
"Importantly, the group also views itself as the now-leader of a global jihadist movement," Olsen said. "It turns out timely, high-quality media, and it uses social media to secure a widespread following."
ISIS has a penchant for producing slick videos that resemble trailers for Hollywood action movies.
One hour-long video showed a collection of bombings, executions, kidnappings and beheadings. As one roadside bomb blasts a vehicle into the sky, two men in the background of the video chuckle.
"We are way behind. They are far superior and advanced than we are when it comes to new media technologies, social media, when it comes to video production qualities, and in disseminating their propaganda over the Internet," said Maajid Nawaz
, a former jihadi and author of "Radical: My Journey out of Islamist Extremism."
And it's probably no accident that a knife-wielding man in the beheading videos of Americans James Foley and Steven Sotloff and Britons David Haines and Alan Henning had a distinctive British accent.
It drives home the point that Westerners can join ISIS, too.
It exerts a cult-like control
When Steve Hassan heard an ISIS propaganda tape, "it reminded me of the exhortations of Jim Jones' cult saying, 'Come to Jonestown. It's a paradise. It's wonderful.' And that keys in on one of the key principles of destructive cults, which is deceptive recruiting."
Hassan is the founder of Freedom of Mind, an organization dedicated to exposing destructive cults and cult behavior.
"This is a political cult using religion and a perversion of Islam as the shield. But in fact it's a systematic effort to create an army of basically tranced-out followers."
Video clips of foreign jihadists burning their passports show that many have no interest in returning home.
It uses its successes as tool
About 2,000 Westerners have gone to fight in Syria, though it was not clear how many joined ISIS in its quest snatch more land and how many joined moderate opposition groups battling the Syrian government, a CIA source told CNN last month.
But it's the extremist groups like ISIS that attract the most foreign fighters, Barrett wrote.
"They tend to be more inclusive, better organized and better financed than their more moderate counterparts," he
said. "They also tend to be more assertive and have more of an impact on the battlefield, and so enjoy greater local standing, which makes them still more attractive to foreign fighters looking to make their own impact."
UK authorities believe at least 500 British citizens have gone to Iraq and Syria, many of them to fight with ISIS and other Islamist groups.
French authorities estimate more than 700 people have traveled from France to fight in Syria, according to Barrett.
On Sunday, FBI Director James Comey told CBS' "60 Minutes" that his agency is tracking a dozen Americans that have joined terrorist groups in Syria.
But it's not all alarming news. Concerned by the prospect of these recruits bringing home their angst and their ability to launch attacks, Western nations are fighting back.
Some European countries have laws that penalize membership in groups such as ISIS, Barrett said.
A U.S. State Department official said an anti-ISIS messaging campaign is keeping disaffected youth from joining.
"We have evidence that there are young people who are not joining because we have somehow interceded," Under Secretary of State for Public Diplomacy and Public Affairs Richard Stengel said Monday.
"They're reading the messages, they're hearing the messages -- not just from us but from the hundreds of Islamic clerics who have said that this is a perversion of Islam, from the hundreds of Islamic scholars who have said the same thing."
But Barrett said elected officials might not be doing as much as they think.
"Policymakers often underestimate the impact of what is happening in these closed circles even as they overestimate the impact of their own."