In the wake of the terror attacks in Paris, France terror raids were carried out in Belgium and Greece to identify, locate and hunt down so-called “sleepers cells.” Journalists and politicians have finally raised the issue of the threats potentially posed by sleeper agents in the United States, going back to the future — the same concerns about sleeper cells in the United States were voiced in the wake of the terror attacks of September 11, 2001 including by the then-director of the FBI, Robert Mueller.
More/Source: https://nationalterroralert.com/sleeper-cells-the-immigration-component-of-the-threat/