The main worry for law enforcement is that U.S. militants will one day return to the United States and attack targets.
Do we have to let them come back here? I would 'think' that anyone who had joined a terrorist group would be automatically nixed from EVER entering the US again. We know one thing IF it was a guy they'd keep him out. Maybe at least on an individual basis .. but cripes that would take much time and money .. I'm for never letting them come home again.
Yes, I feel a tug .. as I believe they are mostly teenagers . .so that is why I say ... maybe on a case by case evaluation.. and I believe we would just have to accept that we would get some of our decisions wrong.. ..