CIA Director John O. Brennan, speaking at a global security forum hosted by the Center for Strategic and International Studies on Monday, warned that the recent attack in Paris wasn't a "one-off event" and that ISIS has other attacks planned.

"I certainly would not consider [Paris] a one-off event. It is clear to me that ISIL has an external agenda that they are determined to carry out these types of attacks," he said, according to The Washington Times.

"This is not something that was done in a matter of days," he asserted. "This is something that was deliberately and carefully planned over the course, I think of several months - in terms of making sure that they had the operatives, the weapons, the explosives with the suicide belts - and so I would anticipate that this is not the only operation that ISIL has in the pipeline."

These events have had experts grappling with how French intelligence could have missed an attack that was likely months in the making.

Brennan noted that while intelligence and counter-terrorism agencies aren't underestimating ISIS, uncovering the specifics of potential become more difficult as terrorist networks become more advanced in their technological capabilities and figuring out ways to communicate without being detected, according to Business Insider.

He also became the latest intelligence expert to suggest that the exposure of national security secrets by leakers have hindered the U.S.' ability to monitor terrorists.

"In the past several years, because of the number of unauthorized disclosures and a lot of hand-wringing over the government's role in the effort to try to uncover these terrorists, there have been policy and other legal changes that make our ability to collectively find these terrorists much more challenging," Brennan asserted, according to Breitbart.

He concluded by recognizing the danger of compromising civil liberties too much in the pursuit of national/global security.

"We don't want these terrorists to succeed in taking away the liberties we pride ourselves on. We should be wary, but we don't want to hermetically seal our borders," Brennan warned. "That is inconsistent with what our societies have been founded on."