Thousands of the world's most wanted jihadists remain a security hot potato for Syria's Kurds, even after Washington announced the death of their elusive "caliph" Abu Bakr al-Baghdadi.
A Turkish attack on northeastern Syria in early October sparked fears the Islamic State group suspects could break out en masse as their Kurdish guards were called up to the front.
A Turkish-Russian truce deal signed on October 22 has since largely halted that offensive, but the fate of suspected Baghdadi followers remains uncertain.
This is what we know:
According to the Kurdish administration, there are around 12,000 suspected IS 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 IS, 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 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.
After Baghdadi was announced killed on Sunday, SDF commander-in-chief Mazloum Abdi warned IS fighters still at large would seek revenge.
"This is why anything is possible, including attacks on prisons," he said.
His warning comes after dozens of IS prisoners were reported to have escaped when the long-threatened Turkish invasion became reality.
Last week, Washington's special envoy for Syria James Jeffrey told Congress "over 100" IS prisoners had escaped and their whereabouts were unknown.
On Friday, Defence Secretary Mark Esper said Kurdish forces had managed to "recollect" dozens of IS prisoners who had escaped during the Turkish assault.
The Kurds had for months been saying that any offensive would make guarding foreign prisoners "a second priority".
Turkey's invasion has lent some added urgency, however, to the search for a 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 neighbouring Iraq.
Apparently anticipating the risk of jailbreaks, the United States took control of two of the most high-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 guards are struggling to control riots.
Another major facility housing so-called "IS families" is Ain Issa, which has found itself in the heart of the battlefield and from which around 800 people escaped in October, Kurdish authorities say.
Some of them are since thought to have been reintegrated in the camp, others to have crossed over to the Turkish side of the front line, and several to have joined up with IS cells operating in the area.
Whether large-scale jailbreaks are prevented or not, any redeployment of SDF fighters away from the detention facilities would create a security vacuum for IS to fill in the area.
The past few months have seen an increase in the number and scope of attacks by IS sleeper cells, that never stopped being active after the jihadist group lost the final fragment of its once sprawling "caliphate" in March.
Thousands of the world's most wanted jihadists remain a security hot potato for Syria's Kurds, even after Washington announced the death of their elusive 'caliph' Abu Bakr al-Baghdadi