Some take videos of themselves burning their Western passports. They say they don’t care if they are stripped of their British or French citizenship. They boast of enslaving Christian and Yazidi women. They decapitate those men deemed foes of their faith and celebrate the gore online, holding up the severed heads. One group of British-born recruits to the so-called “Islamic State” has killed two Americans and one of their own countrymen. They are now threatening to execute another.

Is it too much to say that this al Qaeda breakaway widely known as ISIS or ISIL is really just a collection of serial killers? A vast gang of psychopaths? Certainly that's not the whole explanation for the group’s singular place, so recently and quickly acquired, in this century’s annals of terrorism. But, for sure, it is part of the picture.

For the British-Pakistanis who participated in the horrific beheadings seen round the world there are feelings of reckless exhilaration, according to former FBI profiler Mary Ellen O’Toole. They are likely to be very full of themselves just now: “They have been cultivated to feel important, and taking on the President of the United States and the Prime Minister of Britain in such an aggressive way will fuel that sense of importance.”

For U.S. and European leaders the flow of Western recruits prompts fears of blowback. In explaining his reasons for intervening, President Barack Obama warned Americans starkly that these murderers “could try to return to their home countries and carry out deadly attacks.”

And the recruits themselves are eager to fuel those fears, baiting and raging at the West on their social media accounts and urging those like-minded militants who have not undertaken the hijrah (religious migration) to the “caliphate” to mount “lone-wolf” attacks back home. Or in the words of Aqsa Mahmood, a 20-year-old Glaswegian woman jihadist, “If you cannot make it to the battlefield, then bring the battlefield to yourself.”

Intelligence analysts estimate, roughly, that there are some 3,000 Western recruits in the ISIS ranks, including some 200 women. But the analysts are not confident at all about estimates of how many of those 3,000 veterans of the Syrian and Iraqi battlefields might bring their violence back to the streets of Western Europe or the United States. A Norwegian study found that between 1990 and 2010 one in nine Western militants who fought overseas subsequently became domestic terrorists. Former FBI agent Martin Reardon cautions that “if only a few of them morph from being foreign fighters into terrorists, that will be a big challenge.”

It’s conceivable the alarm about Western recruits is misplaced—at least in the short term. Researchers at the Middle East Media Research Institute, a U.S.-based nonprofit that monitors jihadists, argue in a study of the organization's official writings and speeches that its program differs from al Qaeda. “Unlike Al Qaeda, the IS places priority not on global terrorism, but rather on establishing and consolidating a state, and hence it defers the clash with the West to a much later stage.”

That may be so, but Western leaders and counter-terrorist officials can’t defer their contingency planning until they know if MEMRI is right or not. With the U.S.-led military intervention in Iraq now under way, ISIS may well shift its priorities to focus on the West sooner rather than later. And even without specific direction, people inspired by ISIS propaganda may try to stage terror attacks on their own.

That appears to have been the case with a French killer named Mehdi Nemmouche, who murdered four people at a Jewish museum in Belgium, and a would-be terrorist, Mufid Elfgeeh, arrested by the FBI in New York. What is striking about both those examples is that the men involved were plotting their attacks in sympathy with ISIS long before it conquered Mosul, the second-largest city in Iraq, long before it became the focal point of international fear and anger, long before it began its video campaign of beheadings and, most propaganda spouted by one of its terrified but dignified hostages in what may be a series of videos leading up to his violent death.

Identifying who already has gone to join ISIS, who might go, and why is an urgent priority. The FBI’s psychological analysts at Quantico have been enlisted to chart the pattern of radicalization and to try to get into the heads of these young men and women.

“Different motivations drive different recruits,” says former senior bureau profiler O’Toole. “You can look at second- or third-generation immigrant men and say they are more vulnerable, but one variable doesn’t make a mass murderer.” Real-world profilers have to be careful, and are, not to indulge in facile ethnic, racial or religious “profiling.” Nature and nurture, genetics and family background all come into play. It is as hard as pinpointing potential serial killers from the general population.

What is clear, says O’Toole, is that most Western recruits are “in a similar age range of late teens to mid-20s and that is a problematic age group: It is the age group that we see most of the mass murderers in America coming from, it is the age group where we see the start of mental illness, particularly with males. If males are going to become criminal that is when you start seeing the early manifestations.”

It is the age group of the four British-Pakistani jihadists, dubbed “The Beatles” by Western captives, who appear to have been involved in the murders of American journalists James Foley and Steven Sotloff and British aid worker David Haines. Their leader, nicknamed “Jihad John” in the press, is thought to be an educated Londoner and has been described by a freed Western hostage as intelligent and knowledgeable about radical Islamic teachings.

Misplaced religious beliefs may have been a key factor in the decisions of some of these men to join the global jihad. But not all recruits are well versed in jihadist theology or even the Qur’an. Two British jihadists setting out for Syria ordered Islam For Dummies and The Koran For Dummies before they left.

Islam may be just a vehicle for nihilistic rage. Richard Barrett, a former British intelligence officer who authored a report on Western recruits for the Soufan Group, a security consultancy, says the reasons for Westerners joining the jihad are legion but that “people are seeking a greater purpose and meaning in their lives.” He notes French officials describe their jihadists as “disaffected, aimless and lacking a sense of identity or belonging.”

According to a former Taliban recruiter, Mubin Shaikh, who now works for counter-terrorism research groups, jihadist mentors focus on people who don’t know the religion well and concentrate on converts, as they are more impressionable. Analysts have noted that in Belgium jihadist recruiters focus on the more shabby and dull towns where job opportunities are slimmer and excitement is in short supply.

“Violence is never simple to understand,” says O’Toole. “When you have someone who resorts to this kind of intense violence it is an evolutionary process. They were groomed. And the grooming doesn’t stop once they get there. They may kill because they have to—because it is required of them once they get over there.”

O’Toole doesn’t think all the Western recruits are psychopaths. “That doesn’t make sense to me,” she says. “Most people that young don’t think about consequences. They may never have been violent, although they may have talked a good game back at home. Once they are in Syria they are being exposed to violence that is off the charts and some of them will not be able to handle it.”

That appears to have been the case for a group of 30 British jihadists, who are reported to be disillusioned and eager to return to Britain, according to The Times newspaper. One jihadist claiming to represent the group contacted a de-radicalization center in London saying they were ready to submit to surveillance in the UK, if that meant they could avoid jail terms. “Right now we are being forced to fight—what option do we have?” he told researchers at King’s College London.

Others, though, demonstrate all the hallmarks of being psychopaths, says O’Toole. She would include the British jihadists involved in the killings of the American reporters and Haines. They engaged in what psychologists would describe as “instrumental violence” that “requires you look at people not as human beings but just as objects that you can do with what you want. It is very cold-blooded and very well planned out violence; you kill even though your victim did nothing against you and didn’t deserve it.”