Michael Leiter, the director of the U.S. National Counterterrorism Center has warned that it's inevitable some future terror attacks on American soil will succeed.

Leiter said that's largely due to more Muslims "self-radicalizing" and launching small-scale attacks on their own.

"Just like any other endeavor we will not stop all the attacks," he said. "To say that we will not successfully defend against all attacks is certainly not to say that we are not trying to stop all attacks, we are. It is certainly not to say that any attack is OK. If there is an attack it may well be tragic."

"And we have to be honest that some things will get through," he added. "And in this era of this more complicated threat, a more diverse threat, and lower-scale attacks to include individuals who have been radicalized here in the homeland, stopping all the attacks has become that much harder."

Leiter insisted, though, that terrorism will never be completely successful if Americans are resilient and refuse to be brought down by attacks.