The prospect that thousands of the world's most fanatic militants could break out in the chaos caused by Turkey's invasion of northeastern Syria is causing widespread alarm.
The Kurdish administration guarding those suspected members of the Islamic State group is crumbling and their fate has never seemed more uncertain.
This is what we know about the detainees, whose fate has been a security and diplomatic hot potato for months:
According to the Kurdish administration, there are around 12,000 suspected ISIS fighters in the custody of Kurdish security forces across northeastern Syria.
At least 2,500 of them are non-Iraqi foreigners of more than 50 different nationalities. Tunisia is thought to have the biggest contingent.
Officials in Paris say 60 to 70 French nationals are among those held.
The rest are around 4,000 Syrians and roughly the same number of Iraqis.
The fighters, who were detained mostly in the course of operations led by Kurdish forces and backed by the US-led coalition against ISIS, are detained in at least seven facilities.
The Kurdish-led Syrian Democratic Forces do not reveal the exact locations but some of them are known, including in Roj, in Dashisha, Jerkin, Navkur, Qamishli, and Derik.
Given the high value of some of the detainees, the security levels at these facilities is poor. "They are only buildings" and not heavily fortified, said one top official.
The SDF have warned its erstwhile coalition allies for months that if they needed to mobilize against a Turkish threat, guarding foreign prisoners would become "a second priority".
With the US pulling out of the area, jailbreaks -- a signature component of resurgence drives by ISIS' earlier iterations -- have become a real possibility.
The Kurds said five ISIS suspects escaped from Navkur last week but Washington stressed that no "major breakout" had yet been reported.
France also said Wednesday that Turkey's assault was not yet a threat to the detention facilities' security.
Turkey's invasion has lent some added urgency, however, to the search for a future solution for these prisoners, whom the Kurds warn they cannot keep, let alone prosecute.
Western governments such as France have been reluctant to take them back, for lack of a clear legal framework and fears of a public backlash.
France and other governments have sought instead to transfer some of them to neighboring Iraq, an option French Foreign Minister Jean-Yves Le Drian was expected to discuss in Baghdad this week.
Apparently anticipating the risk of jailbreaks, the United States took control of two of the most-profile IS detainees in the early hours of the Turkish offensive and spirited them out of the country.
The detained fighters have thousands of relatives -- mostly women and children -- held in other facilities, such as the infamous Al-Hol camp, which is so overcrowded that wardens are struggling to control riots.
Another major facility housing so-called "ISIS families" is Ain Issa, which has found itself in the heart of the battlefield and from which around 800 people escaped on Sunday, Kurdish authorities say.
Some of them are since thought to have ben reintegrated in the camp, others to have crossed over to the Turkish side of the front line and others to have joind up with ISIS cells operating in the area.
As recently as Tuesday, a breakout attempt was foiled in Al-Hol, an SDF official said.
Whether large-scale jailbreaks are prevented or not, the redeployment of SDF fighters away from the detention facilities to defend against the Turkish assault creates a security vacuum for ISIS to fill in the area.
The past few months have seen an increase in the number and scope of attacks by ISIS sleeper cells, that never stopped being active after the militant group lost the final fragment of its once sprawling "caliphate" in March.