(VOVworld) – US national security is facing unprecedented challenges—potential jihads—when American citizens who have fought alongside with extremist groups in Syria and Iraq have returned home.

The Pentagon said initially only about a dozen Americans have gone overseas to join up with ISIS, but now the administration concedes that some of them have returned successfully and are currently in the US.

An anonymous American official said on Monday that some of the 100 individuals suspected by FBI had returned to the US and are under close surveillance. Latest statistics indicate that approximately 15,000 individuals, including some 2,000 European citizens, in 80 countries intended to go or have gone to Syria and Iraq to become jihadists.