This has been a long-standing concern, as those fighters will eventually return home with ties to international "jihadist movements", and an eye toward attacks in the Western world.
Wainwright warned Europe faces a “capability gap” in dealing with the returning terrorists, and that the risk of sleeper cells was more than at any time since 9/11.
Europeans have flocked to Syria, mostly to join ISIS, in numbers unprecedented, and ISIS and the other groups have been able to absorb them with relative ease.
Some 1,000 terrorists head to Syria and Iraq from around the world during any given month now, as the US-led war makes it even easier for ISIS to recruit from abroad.