"I certainly would not consider it a one-off event," said CIA Director John Brennan, speaking at a Center for Strategic and International Studies forum in Washington on Monday.
"It is clear to me that ISIL has an external agenda, that they are determined to carry out these types of attacks," he continued, using another term for ISIS. "This is not something that was done in a matter of days. This was carefully planned over the course I think of several months, in terms of making (sure) they had the operatives, the weapons and the explosives with the suicide belts."
He concluded, "And so I would anticipate this is not the only operation that ISIL has in the pipeline."
ISIS members have worked hard to learn new ways to conceal their tactics from Western law enforcement entities, according to Brennan.
"There has been a significant increase in the operational security in the number of these operatives in these terrorist networks because they have gone to school on what it is they need to do to keep their activities concealed from the authorities," he said.
The comments from Brennan came the day after members of the President's national security team said that while ISIS certainly has the ambition to launch similar attacks on U.S. shores, the capability is not great..
Deputy National Security Adviser Ben Rhodes told reporters that one big difference between the situation in Europe and that in the U.S. is that "thousands" of fighters have traveled to Syria and then returned to Europe. The number being tracked in America is far smaller -- around 40, according to an estimate by Director of National Intelligence James Clapper earlier this year.
Clapper added that not all of those ISIS joiners were necessarily fighting -- some might have served other roles for the terrorist group, such as first aid -- and he knew of no terrorist plots that any of those returning have been involved in once back in the U.S.
Rhodes said that ISIS has found more utility in trying to recruit or motivate sympathizers in America online from abroad.
A top FBI counterterrorism official told Congress this year that "hundreds, maybe thousands" of people in the U.S. follow ISIS online. And this year alone, at least 49 alleged ISIS "supporters" in America have been charged with related crimes. The largest number of those were in New York.
An ISIS-inspired plot in Garland, Texas
, in May, in which two men with body armor and assault rifles opened fire outside an art contest in which participants drew pictures of the prophet Mohammed, was thwarted by a security officer who shot and killed both suspects.
The FBI had been watching one of the suspects' online activity referencing the contest, and had warned police in Garland hours before the attack, though officials didn't know he was planning an attack, and weren't closely monitoring his physical whereabouts.
Following that attack, intelligence officials have said they are now doing more monitoring of "hundreds" of suspected ISIS supporters.
Rhodes emphasized Sunday there is currently "no specific, credible threat" of an attack being planned in the U.S., but vigilance among law enforcement is high.