Speaking on the channel’s 60 Minutes program, which was broadcast on Sunday, Brennan said that the agency and its allies were aware of covert ISIS activity in the lead up to the attacks in the French capital, however attempts to stop it was foiled by “sophisticated” use of encrypted Internet communication.

When asked what he and his colleagues have learned from the November attacks, Brennan said the attacks showed “that there is a lot that ISIL [an alternative acronym for ISIS] probably has underway that we don’t have obviously full insight into.”

“We knew the system was blinking red,” Brennan said “We knew just in the days before that ISIL was trying to carry out something. But the individuals involved have been able to take advantage of the newly available means of communication that are walled off from law enforcement officials.”