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.

You have to keep in mind that there's a number of logistical hurdles that they have to jump through in Europe in order to get explosives or guns. So for example, the Villejuif shooter in April 2015 who planned to attack a church: According to French press reports, he was instructed by ISIS to go to a sandwich shop, and the weapons would be waiting for him in a car parked outside. He was just told the make of the car, told to go there, told to pick them up. Putting that kind of attack together, using encrypted communications or other means, is not easy.

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.

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.

[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.

I'm really curious as to the actual structure of the Emni [the intelligence unit that handles foreign attacks]. And Abu Muhammad al-Adnani, the head of it. There are so few pictures of him. We don't even really know what he looks like. For somebody with this outsized influence projecting murder abroad, you'd think that we would know a lot more by now. And then the role of the hostages.

I've had countless jihadis shut me down straightaway and say, "I'm not allowed to speak to any woman who is not my relative." But among the few I'm able to get through to, I think that being female makes me more approachable, soft, all the stereotypes of what female means.