First, I would say, is ideology. I have never spoken to any member of these groups, not just ISIS, but also Al Qaeda, Shabab, etc., who wasn't driven by the ideology. Beyond that, a lot of people are wounded in some way - they've fallen out of society in some manner. All of the ones I've spoken to seem to have veered off course from lives we consider acceptable and successful. Of course, there are others like Osama bin Laden, who was a wealthy and successful businessperson.

One of ISIS' biggest propaganda coups was the beheadings of the aid workers and journalists. Is [Emni], the group that is exporting fighters overseas, also the one that was holding James Foley and John Cantlie and Kayla Mueller?

[People who have left the ISIS] say that there comes a moment when the inconsistencies and apparent hypocrisies of their sheikh lets them down, and they begin rereading scripture and find ways that vouch for a nonliteralist reading of the Koran.

People who have left the group talk about how a religious inspiration took them to ISIS. It was their feeling of being marginalized as Muslims in the society where they were living, and then buying into the promise of a caliphate and of a Muslim land that is governed as in the time of the prophet. I have yet to meet anybody, or speak to anybody, who was not religiously motivated at some level.

We know that Muhammad waged war against the Qurayshi tribe, his own tribe, and it's from that conflict that much of the concept of jihad and verses that ISIS now uses to justify beheadings come from. A young man just told me that he went back and read this carefully [and saw] the prophet and his people were fighting the Quraysh because they were not allowing the prophet and his people to practice their religion.

It seems to me that a 9/11-style attack is impractical.

It's become extremely difficult to speak to active ISIS members now. I was speaking to them in 2015, and the last one was killed in a drone strike.