"Paris was a failure of intelligence. All but one of the eight terrorists were French citizens, trained by ISIS in Syria. They returned, unnoticed, and attacked six locations killing 130 people," Brennan said in an interview.
"There is a lot that ISIL probably has underway that we don't have obviously full insight into. We knew the system was blinking red. 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 – that are walled off, from law enforcement officials," he said.
Read alsoParis attacker "slipped" into EU with 90 other terrorists: WSJAccording to Brennan, CIA should work harder. "We gotta work harder. We have to work harder. We need to have the capabilities, the technical capabilities, the human sources. We need to be able to have advanced notice about this so that we can take this – the steps to stop them. Believe me, intelligence security services have stopped numerous attacks – operatives – that have been moved from maybe the Iraq to Syria theater into Europe. They have been stopped and interdicted and arrested and detained and debriefed because of very, very good intelligence."
Read alsoU.S. intelligence chief warns of 'homegrown' security threat"The ISIS assault on Paris and the ISIS-inspired massacre in San Bernardino, California, share a disturbing fact, no one saw them coming. Today, the biggest terrorist threat to the United States is not like al Qaeda. ISIS is wealthy, agile, sophisticated online, and operates freely in a vast territory of its own... it has the manpower, means and ruthlessness to attack the U.S," CIA Chief said.