John Brennan, the director of the Central Intelligence Agency, revealed during an interview at CBS's "60 Minutes" that a massive attack by ISIS on the U.S. is bound to happen. He also said that the CIA knew that the terrorist group was planning an attack on Paris days before the mass shooting in the city occurred last year started.

According to Brennan, previous incidents, such as the shooting in San Bernardino, already indicate that members of ISIS have already secretly infiltrated the U.S. And, just like what one of the suspects in that incident, did, they may already be spreading pro-terrorist or anti-U.S. propaganda within the country through various means such as the Internet, CBS News reported.

For the CIA director, these actions are considered as part of the extremist group's planning process in carrying out a massive attack on the country. If given enough time and resources, Brennan believes ISIS fighters will be able to execute this plan eventually.

He noted that this is similar to what happened in 2015 during the Paris attacks that left 130 people dead. Although he admitted that the CIA already knew that the terrorists were planning that attack days before it happened, the agency could not exactly pinpoint as to where it will take place.

A major factor that prevented CIA operatives from zeroing in on the actual plans of the extremists was their use of encryption technology, which prevented the agency from accessing vital information about the suspects' actions, according to HNGN.

Brennan said that in order to prevent the same incident from happening in the U.S., authorities need to step up their efforts.

"We gotta work harder," he said during the interview. "We have to work harder. We need to have the capabilities, the technical capabilities, the human resources. We need to be able to have advanced notice about this so that we can take this - the steps to stop them."

Brennan then went on to say that members and supporters of ISIS have already tried to carry out small-scale attacks in the U.S., but most of these were not publicized since they were immediately stopped by authorities before they happened.

The revelations made by Brennan highlights the need for the U.S. government to come up with a forward-looking strategy to protect the public and suppress terrorist threat. Instead of employing reactive operations, agencies must be able to come up with strategies that can stop terrorists in their tracks before they get to carry out what it is that they're planning to do.