Even the defeat of the Islamic State (IS) militants in Iraq and Syria will not extinguish the extremist group's threat to the civilized world, top U.S. intelligence officials say.
"The threat that I think will dominate the next five years for the FBI will be the impact of the crushing of the caliphate, which will happen," FBI Director James Comey told a conference in Washington on September 8.
"Through the fingers of that crush are going to come hundreds of hardened killers, who are not going to die on the battlefield. They are going to flow out."
Comey predicted that many will head into Western Europe and try to duplicate recent attacks in Paris and Brussels, while others will bring the fight to the United States.
CIA Director John Brennan agreed that many fighters who don't die on the battlefield will try to return to their native countries and launch attacks.
He noted that Al-Qaeda in Iraq was reduced to several hundred fighters at one point, yet was able to reemerge as IS.
"I think a number of them are going to remain a challenge for the United States and other governments for a number of years to come," he said.