After the Boston Marathon, San Bernadino, and Orlando events, rather than bringing in operatives to create 'sleeper' cells, the practice seems to be the recruitment of children of immigrants that are sympathetic to Islamic jihad.
It eliminates the the logistics and operational risks of getting someone into the country and simplifies everything to include security (no new neighbors to call attention to the operation when you can get the kid that grew up there).
PBS did a story on European Immigrants and their children's inclination to radicalize. The same trend could be true here:
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