Top U.S. counterterror and law enforcement officials are offering grim warnings about what awaits once the Islamic State terror group's self-declared caliphate ultimately collapses.
Much of the U.S. and coalition strategy against IS has been predicated on the idea that without the ability to hold territory in Iraq and Syria, the terror group will falter, unable to make good on its promise of a utopian society.
Yet as the fall of the IS caliphate looks to draw nearer, U.S. officials are increasingly cautious, saying the group will remain a dynamic and formidable threat for some time to come.
"We all know there will be a terrorist diaspora out of the caliphate," FBI Director James Comey told the House Homeland Security Committee on Thursday.
"Those thousands of fighters are going to go someplace," he said. "Our job is to spot them and stop them before they come to the United States to harm innocent people."
As many as 40,000 foreign fighters from more than 120 countries are believed to have flocked to the conflict in Syria, with a majority of them joining IS while the group grew and saw its fortunes rise as it advanced across Iraq.
Now, Comey and others worry the terror group is in a prime position to take advantage of the flow home.
FILE - A man sits amid a makeshift memorial inside a burned mall at the scene of a massive truck bombing last Sunday that killed at least 186 people and was claimed by the Islamic State group, in the Karada neighborhood of Baghdad, Iraq, July 10, 2016.
"ISIL's external operations has been building and entrenching during the past two years," according to National Counterterrorism Center Director Nicholas Rasmussen. ISIL is an acronym for Islamic State.
"We don't think that battlefield reverses alone in Iraq will be sufficient to degrade that terrorism capability," he said. "Over time we will degrade their capability … but there may be a significant lag."
Adding to the concerns is the terror group's use of forged documents to get its operatives into Europe, where some became key players in the terror attacks that rocked Paris and Brussels.
"We know it's a part of ISIL's tradecraft," Comey said.
U.S. Homeland Security officials say they have bolstered the department's abilities to detect fraudulent travel documents, though the risk remains that an IS operative could find a way to sneak into the U.S.
Concerns also persist about the challenges facing some U.S. allies, especially in Europe where, despite ongoing crackdowns, intelligence officials fear IS has become "deeply rooted," using perhaps hundreds of jihadists who returned from Syria and Iraq to mold thousands of would-be foreign fighters into guerilla units.
"The Europeans have made progress since the horrific attacks in Paris and Belgium, but clearly more progress has to be made," CIA Director John Brennan said Wednesday during an appearance at the Brookings Institution.
"Some of the countries themselves, their internal and external services, will not talk with one another; they're not knitted together," he said.
Just getting intelligence on IS is proving to be problematic.
FILE - Iraqi security forces inspect vehicles belonging to escaping Islamic State militants outside Fallujah, Iraq, Wednesday, June 29, 2016.
"ISIL is a savvy, experienced adversary that knows how we collect intelligence," Rasmussen said Thursday. "I would describe our efforts to gain an understanding of ISIL intentions and strategy and direction as being a harder target right now than what we faced with al-Qaida."
One area where officials say they have made good progress is in efforts to prevent U.S. citizens from traveling to join IS. But even there, concerns persist as officials worry some may be heeding the terror group's call to stay home and find ways to carry out attacks where they live.
The threat of attacks inspired by IS messaging on social media is also increasing.
FBI officials have said they are currently investigating about 1,000 such cases, but face difficulties as many of the would-be terrorists are not actively communicating with other sympathizers or operatives.
"The prospect of homegrown violent extremism — another San Bernardino, another Orlando — is number one on my list," Homeland Security Secretary Jeh Johnson told lawmakers Thursday when asked what keeps him up at night.
"We are not winning the war against Islamist terror," House Homeland Chair Rep. Michael McCaul said Thursday.
"Each day we stick with half measures, ISIS is able to dig in further and advance a murderous agenda," he said, using another acronym for the terror group. "Overall they are not on the run, they are on the rise."