"They do have that capacity," Director of National Intelligence James Clapper told CNN's Peter Bergen in an exclusive interviews on "AC 360" on terrorism, Osama bin Laden and al Qaeda's most virulent offshoot -- ISIS.
"That's something we worry about a lot in the United States, that they could conjure up a raid like they did in Paris or Brussels," where March attacks on a train and at an airport left 32 dead and 300 people injured, Clapper said. The November Paris attacks killed at least 130.
However, President Barack Obama and some of his other security advisors spoke of the threat in less stark terms and emphasized efforts to protect the U.S.
Obama told Bergen that "we, here in the United States, face less of a threat than Europe" from ISIS. National Security Adviser Susan Rice said "whether or not" ISIS can attack the U.S., the administration would do "our utmost to try to prevent it."
Still, Obama said, "The Paris-style attack, the Brussels style attack is the challenge that we're going to continue to face."
Obama and his innermost security team, including then-Secretary of State Hillary Clinton, now the frontrunner in the Democratic presidential race, discussed ISIS's emergence, threat and long-term prospects over three hours with Bergen. They called for increased action against the group and forecast an effort that could take decades. They also offered one-word portraits to sum up their view of the jihadist group and its global reach.
ISIS is 'opportunistic'
Homeland Security Advisor Lisa Monaco described ISIS as "opportunistic." White House Chief of Staff Denis McDonough called it "present" -- only able to prove its relevance through destruction and "by virtue of its nefariousness and threat."
To Clinton, the group is "evil." She described it as "an evil bunch of criminal thugs who hide in the cloak of religion to dominate" and seize territory, bank accounts, oil and oil production facilities.
Clapper offered the word "brutality." He predicted that ISIS' beheadings, crucifixions and repression would eventually be self-defeating.
In the meantime, he and other administration officials raised their concerns about attacks like the broad assaults ISIS unleashed in the streets of Paris and Brussels.
Clapper said any attempted attack in the U.S. would echo these assaults in Europe and, as in those cities, ISIS would "either infiltrate people or incite people who are already here," he said. In a reference to the December shootings in San Bernardino, California, he added that, "we've already seen some cases of that."
ISIS central would not necessarily give directions for a specific target, Clapper added. "That's apparently not exactly their modus operandi," he said. "It's more general, strategic guidance, and then let the local cell figure out how to achieve the objectives."
"There is a very determined element of ISIS that is trying to carry out attacks abroad as they've done in Belgium and Paris and other areas," said John Brennan, director of the Central Intelligence Agency.
Clapper qualified the level of danger ISIS poses to the U.S., saying that, "as an existential threat to this country, it's not ISIS or any other other terrorist group." But he said that he also worries about the "lesser-scaled attacks of what we have seen in this country, or worse, a Paris or Brussels kind of thing."
That possibility means that the U.S. should dintensify its efforts against ISIS, Brennan said. "I do believe that we need to add some urgency to this destruction of ISIS," the CIA director said, "just because it has demonstrated that it's willing to carry out these horrific attacks against innocent civilians."
Obama and many of his advisers said it will be a long fight. ISIS reflects an ideology that made its bloody debut on the global stage with bin Laden, they said, and has since spread tentacles around the world.
"Even while bin Laden was still alive, the ideology that he put forward, the methodology that he tried to spread, metastasized in some fashion," Obama said.
Among a range of terrorist groups that sprang up, Obama said some had more capacity than others, some more regionally focused and some focused on engaging in attacks in the West or on the homeland.
Obama calls for vigilance
"We have to remain vigilant," Obama said.
Rice's word was "2.0" -- a reference to the way ISIS has taken al Qaeda's basic ideology and morphed it, most obviously projecting themselves into cyberspace. "It's utilized the Internet and social media to recruit in a way that al Qaeda never did," Rice said.
Clapper assessed that there are now eight provinces of the so-called Caliphate that ISIS has claimed to have established and several other "nascent provinces that are striving, it appears, to be considered full-up members of the Caliphate."
That spread -- online and in the real world -- has its roots in conditions the U.S. has little power to sway in the near term, the officials said.
Clapper spoke of "fundamental economic, socio-economic conditions" that choke off opportunity and spawn resentment and make for "fertile recruiting grounds." He also pointed to the spread of ungoverned areas where terrorist groups can establish a base from which to train and plot.
"Until those fundamental issues are more effectively addressed," he said, "I think we're going to be in this battle for a long time, whether it's ISIL or some other groups."
Because of those deeply rooted underlying factors, other groups may branch off from ISIS the way ISIS morphed out of an al Qaeda affiliate, the officials said. That means the long-term challenge for the U.S. and its allies will be to keep these groups in check.
"I think we're going to be in a perpetual state of suppression" for these groups, Clapper said.
For now, the administration has been in "relentless pursuit" of ISIS, Brennan said, and is making continued progress.
Clinton touted the administration's efforts against the group, which include working with a coalition of more than 60 countries to attack ISIS' financial and oil infrastructure, stem the flow of foreign fighters joining them in Syria and Iraq, and train Iraqi and Kurdish soldiers to wage the ground battle against them.
But Brennan acknowledged that "the phenomenon of ISIS is going to be with us into the next administration" and predicted that defeating it would probably be "a multi-year effort."
That said, Brennan's one-word description for ISIS was definitive.
"Doomed," he said. "It's just a matter of time."