In remarks prepared for the Senate Intelligence Committee, Brennan says IS has been working to build an apparatus to direct and inspire attacks against its foreign enemies, as in the recent attacks in Paris and Brussels — ones the CIA believes were directed by IS leaders.
"ISIL has a large cadre of Western fighters who could potentially serve as operatives for attacks in the West," Brennan said, using another acronym for the group. He said IS probably is working to smuggle them into countries, perhaps among refugee flows or through legitimate means of travel.
Brennan also noted the group's call for followers to conduct so-called lone-wolf attacks in their home countries. He called last week's attack in Orlando a "heinous act of wanton violence" and an "assault on the values of openness and tolerance" that define the United States as a nation.
He said IS is gradually cultivating its various branches into an interconnected network. The branch in Libya is likely the most advanced and most dangerous, but IS is trying to increase its influence in Africa, he said. The IS branch in the Sinai has become the "most active and capable terrorist group in Egypt," attacking the Egyptian military and government targets in addition to foreigners and tourists, such as the downing of a Russian passenger jet last October.
Other branches have struggled to gain traction, he says. "The Yemen branch, for instance, has been riven with factionalism. And the Afghanistan-Pakistan branch has struggled to maintain its cohesion, in part because of competition with the Taliban."
He called IS a "formidable adversary," but said the U.S.-led coalition has made progress combatting the group, which has had to surrender large swaths of territory in Iraq and Syria and has lost some of its leaders in airstrikes. IS has struggled to replenish its ranks of fighters, Brennan said, because fewer of them are traveling to Syria and others have defected.