U.S. counterterrorism officials are increasingly worried Islamic State fighters captured as the terror group's caliphate collapsed in Syria will find their way back to the battlefield.
The concern, they say, is most acute for the approximately 2,000 foreign fighters who are being kept in a state of limbo, held in makeshift prisons run by the U.S.-backed Syrian Democratic Forces as their home countries refuse to take them back.
"We've gotten kind of fatalistic about this," Russell Travers, acting director of the U.S. National Counterterrorism Center, said Friday.
"There's a growing likelihood that eventually we could see many of these foreign fighters again when they've broken out of prison or been released," he told an audience at the Washington Institute for Near East Policy. "The greatest midterm concern is the retention of those prisoners and not bolstering the [Islamic State] ranks and not seeing a foreign fighter outflow from Syria."
For months, U.S. officials at the State Department and the Pentagon have urged countries, especially those who joined the coalition to defeat IS, to repatriate and prosecute their citizens or residents who left to fight for the self-declared caliphate.
But those pleas have largely gone nowhere, as European countries especially have raised concerns, arguing their legal systems will not allow for the successful prosecution of IS fighters whose alleged crimes were committed thousands of kilometers away.
Making matters more precarious, Turkey's incursions into northeast Syria last month forced the main U.S. partner on the ground, the mostly Kurdish SDF, to pull guards from the prisons to fight Turkish-backed forces.
Kurdish officials also said they were forced to evacuate some prisons and move captured IS fighters to makeshift facilities after some areas came under attack.
In the process, about 100 prisoners escaped. And there are fears that if Turkish forces drive deeper into Syrian territory, eventually there may not be anyone left to keep the remaining IS detainees under lock and key.
The question is a critical one for U.S. counterterrorism officials, who believe there are likely a total of 15,000 IS fighters in SDF custody, thousands more than suggested by previous estimates from U.S. military or diplomatic officials.
Should a significant portion get free, it could significantly alter the battlefield in Syria and Iraq, home now to at least 14,000 IS fighters.
"We think it's substantially higher than that," Travers said, adding, "That number is going to do nothing but grow."
"There are already no-go areas at night. We see ISIS flags and we see small areas in which Sharia is being implemented," he added, using an acronym for the terror group. "The insurgency is alive and kicking."
The United States is not alone in its concern.
Officials from Iraqi Kurdistan have likewise been voicing alarm at the developments in Syria, worried that they could give the Islamic State insurgency a significant boost and give hope and inspiration to IS family members languishing in displaced-person camps across Syria and Iraq.
"These camps are a breeding ground. It's a ticking time bomb," said Bayan Sami Rahman, the Kurdistan Regional Government representative to the U.S.
"You have children growing up in this kind of atmosphere where the parents are unrepentant ISIS fighters, the mother and the father," she told VOA. "I'm worried that in 10 years' time, you and I will be having a conversation about a 5-year-old, who, by then, is 15 in these camps. And what will his mindset be?"
According to U.S. counterterrorism officials, the answer could come down, in part, to how IS decides to operate.
For now, officials say the terror group has played to expectations, rallying support around its new leader, Abu Ibrahim al-Hashimi al-Qurashi, after a U.S. raid killed former caliph Abu Bakr al-Baghdadi.
What comes next, though, may not be so easy to predict.
"ISIS is a learning organization," said Travers of the National Counterterrorism Center. "In my own mind, I wonder if they would be content with conducting a prolonged insurgency and staying underground to avoid the kind of pressure they absorbed from the coalition.
"The more we draw down [U.S. forces], the more we siphon resources off to other very high-priority threats, the greater the likelihood we're not going to understand that dynamic," he said.