The prospect that thousands of the world's most fanatic jihadists 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:
How many?
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.
Where?
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 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.