National Counter terrorism Center Director Michael Leiter said this after the FBI arrested Mohamed Osman Mohamud, a Somali-born US national accused of planning to bomb a Christmas tree lighting ceremony in Portland.
"We aim for perfection, but perfection will not be achieved. Innocent lives will be lost," the ABC News quoted Leiter's saying.
"Just like any other endeavor we will not stop all the attacks. 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. It is certainly, not to say that any attack is OK. If there is an attack it may be tragic. We have to be honest that some things will get through."
"In this era of a 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," he added.
Leiter further underscored that, many Americans like Osman Mohamud find inspiration online from Yemen based American cleric Anwar Al-Awlaki.
He told, "Al-Awlaki as a dangerous threat, who is directly involved in planning attacks on the US."