Every step the U.S. and its coalition partners take toward routing the Islamic State terror group in Iraq and Syria brings with it an ever-growing threat for a new wave of attacks in the West, the director of the U.S. Federal Bureau of Investigation said Tuesday.
"The so-called caliphate will be crushed," James Comey told the Senate Homeland Security and Governmental Affairs Committee. "The challenge will be that through the fingers of that crush are going to come hundreds and hundreds of very dangerous people."
The testimony echoed similar warnings issued over the past several months by Comey and others about a looming "terror diaspora" in which foreign fighters who once flocked to the Islamic State's self-declared caliphate turn their wrath on their homelands.
"They will not all die on the battlefield in Syria and Iraq," Comey said. "There will be a terrorist diaspora sometime in the next two to five years like we've never seen before."
Wrecking the caliphate
U.S. policymakers have long seen destroying the terror group's caliphate, and wrecking its aura of invincibility, as the key to reducing its potency as a terror group.
Senior intelligence and counterterror officials believe that is still true, but the director of the National Counterterrorism Center cautioned that the strategy would take time and would lead to a period of "sustained vulnerability" for the U.S. and its allies.
"The effects we're looking to see are simply going to be delayed or lag behind the physical progress on the battlefield," NCTC's Nicholas Rasmussen told the Senate committee.
FILE - Iraqi security forces patrol as smoke rises from burning oil wells in Qayara, south of Mosul, Iraq, Aug. 31, 2016.
"It's not simply a matter of taking territory or winning a battle in a place like Mosul or al-Raqqa," he said. "That [IS external operations] infrastructure that was set into motion or put into place is going to have to be hunted down and destroyed systematically."
U.S. intelligence officials estimate that more than 40,000 foreign fighters, including as many as 7,600 Westerners, have flocked to the conflict in Syria and Iraq, with a majority of them joining IS.
"When ISIL is reduced to an insurgency and those killers flow out, they will try to come to Western Europe and try to come here to kill innocent people," Comey said, using an acronym for Islamic State. "We must prepare ourselves."
Lessons of 9/11
U.S. and European officials say efforts to coordinate intelligence are well under way, with a focus on helping European countries learn from the American experience in the aftermath of the Sept. 11, 2001, terror attacks.
"Certainly, in the short term it's not going to be better," Dick Schoof, Dutch national coordinator for security and counterterrorism, told a U.S. audience in August. "We have an increased attention for people returning from Syria back to the Netherlands and also back to Europe."
Just how many former IS fighters are likely to return to their countries of origin is not known. But of the estimated 18,000 to 25,000 fighters still in Iraq and Syria, Comey predicted "at least hundreds" would survive.
And both U.S. and European officials caution that some already have returned, either as part of terror cells or with the purpose of creating new ones.
"The [terror] diaspora has already begun," said Republican Senator Ron Johnson of Wisconsin, chairman of the Homeland Security and Governmental Affairs Committee.
"They have to be destroyed," he added of Islamic State's forces. "We can't just nibble around the edges."
FILE - With tanks in the background, youths set fire to barricades in Diyarbakir, Turkey, hours after Kurds protesting the Islamic State advance on Kobani, Syria, had clashed with police, Oct. 8, 2014.
Equally concerning to U.S. and European officials is the notion that the idea of Islamic State will live on long after the caliphate and even the insurgency are dead.
"The prospect of the next terrorist-inspired attack on our homeland is the thing that keeps us up at night," Homeland Security Secretary Jeh Johnson told lawmakers Tuesday.
But some U.S. lawmakers said too many potential threats were still evading law enforcement officials.
"One common attribute to many of the recent attacks is that they were previously investigated by the FBI and they were found not to be credible threats," said Republican Senator Rand Paul of Kentucky, citing the suspects in the Boston Marathon bombing, the Orlando nightclub shooting and this month's bombings in New York and New Jersey.
"I'm not here to say, 'Gosh, the FBI is terrible because it missed these things,' but I'm also troubled that the FBI is unwilling to even admit that they may have made some mistakes," Paul said.
"As long as I'm director of the FBI, we will stare back very carefully at what we do," Comey replied. "Where we make mistakes, we will admit them. We will be transparent and we will get better."