Police officers responding to a suspicious vehicle near La Carillon restaurant on Sunday after a series of deadly attacks in Paris. REUTERS/Pascal Rossignol CIA director John Brennan said he thought a coordinated series of terrorist attacks in Paris on Friday night, which has since been linked to the Islamic State, was probably "not a one-off event."

"ISIS has an external agenda that they are determined to carry out," Brennan said in remarks at the Center for Strategic and International Studies' Global Security Forum on Monday.

"So I would anticipate this is not the only operation ISIL has in the pipeline."

Brennan's comments come after terrorists linked to the Islamic State group (also known as ISIS or ISIL) killed at least 129 people in a wave of attacks throughout Paris on Friday night and into Saturday morning.

The events, which Brennan described as "carefully and deliberately planned," have had experts grappling with how French intelligence could have missed an attack that was most likely months in the making.

"How was this allowed to happen? What went wrong?" Josh Rogin, a reporter for Bloomberg View, asked Brennan on Monday.

Brennan responded by saying that, though many terrorist plots are uncovered and thwarted before they can be carried out, the attack in Paris was clearly a "sophisticated" effort that was "underway for quite some time."

CIA director John Brennan at the John F. Kennedy School of Government at Harvard University in Cambridge, Massachusetts. Thomson Reuters

"It was not a surprise this attack was carried out," Brennan said. "We had strategic warning. We knew that planning by ISIL was underway."

"We are not at all underestimating ISIL's capabilities," he added.

He noted, however, that uncovering the specifics of potential plots has become more difficult as terrorist networks become more sophisticated in their technological capabilities and figure out ways to evade detection as they communicate.

Business Insider's Armin Rosen noted on Saturday that the Paris attack was especially alarming because it "was planned and executed within the capital of a country with a highly advanced anti-terrorism infrastructure."

France was already on high alert after January's attacks on the satirical magazine Charlie Hebdo and a kosher supermarket in Paris.

Moreover, Rosen noted, the attack was executed hundreds of miles from the group's haven and self-proclaimed caliphate in parts of Iraq and Syria.

A man paying respects outside Le Carillon restaurant the morning after a series of deadly attacks in Paris. Thomson Reuters "The fact that an attack this big occurred suggests to me an erosion in surveillance capabilities compared to magnitude of the threat," Daveed Gartenstein-Ross, a senior fellow at the Foundation for Defense of Democracies, told Business Insider on Saturday.

"A few years ago people hoped the age of mass-casualty incidents in Western states was gone because surveillance or interruptions from authorities could prevent attacks like this," Gartenstein-Ross said. "This has put a definitive end to those hopes."

When asked whether the public should begin to accept that these attacks may now be inevitable, Brennan was resolute in his response.

"I would never say these types of attacks are inevitable," Brennan said, noting that intelligence officials were working "tirelessly" to prevent terrorist attacks.

On Saturday, French President Francois Hollande vowed that France would "be merciless to the barbarians of the Islamic State group," which took responsibility for the attacks late Friday evening. The next day, French fighter jets launched their biggest raids in Syria to date, targeting the Islamic State's de facto capital of Raqqa with the help of US intelligence.

Police officers standing guard in Place de la Republique on Sunday. REUTERS/Pascal Rossignol When asked whether he thought Europe should tighten its borders to stop the flow of refugees who might have extremists among them, Brennan was again resolute.

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

US President Barack Obama echoed this sentiment in a statement from the G-20 summit in Turkey on Monday.

"Slamming the door in their faces would be a betrayal of our values," he said of refugees.