The struggle with people who have gone to Syria to join ISIS is not so much the ones you know about, it's the ones you don't know about. That was the message in a briefing by FBI Director James Comey.
Comey reiterated that the US has around a dozen American ISIS members "that we have a pretty good handle on," but conceded that there were potentially many more that they don't know about.
How many? It's not clear, but Comey says that 150 Americans who have gone to Syria in recent months are under surveillance just in case. That doubtless includes innocent people.
But even surveilling everyone they know went to Syria cannot be all-encompassing, as many Western jihadists have actually traveled to Turkey and then snuck into Syria from there.