(CNN) What do you do with 400 of the world's most unwanted extremists, held in a territory that isn't technically a state, that doesn't really have the legal means to extradite them, or the resources to jail them indefinitely?
That's the question facing Syrian Kurdish forces and their US allies in northern Syria now.
The final stages of the anti-ISIS battle swept up a large number of ISIS fighters, including many foreigners. Some may have been allowed to leave Raqqa, the so-called capital of the caliphate, in the final deal agreed between the Syrian Kurds and ISIS to reduce civilian casualties, under which dozens of ISIS fighters, foreign and Syrian, fled with civilians into the desert.
Other ISIS fighters have been on the run longer. Some are unknown players, but some are also noted criminals, like the so-called "Beatles" -- British ISIS fighters who taunted western audiences as they tortured and executed bound, unarmed hostages kneeling before them.
Their fate has presented the US -- to whom much of the world looks for a solution to this problem -- with a quandary.
The US principal deputy assistant secretary of defense for international security affairs, Katie Wheelbarger, recently told reporters: "These aren't necessarily the best detention facilities in the sense of they are being held in Syria and not in the most secure area. I think it would be better if we make sure they are prosecuted if possible in their countries of origin." To this end, talks with other members of the coalition were ongoing, she said.
But the US cannot force other countries to accept their citizens back. Some do not want their extremists.
Notably, five days after their capture, the UK has made no public statement about what it wants to do with the two surviving "Beatles," named by US intelligence sources as El Shafee Elsheikh and Alexanda Kotey. There is clearly a reluctance to put them on public trial in the UK. Perhaps it is out of a fear that it would give them a platform for their extremist beliefs. But there is also a risk of failure -- it may be hard to gather convincing evidence of crimes committed in a far away land. The remains of many victims have yet to be located.
A US official says there has been no discussion about bringing Elsheikh and Kotey to Guantanamo Bay, the American detention facility in Cuba that is frowned upon by UK politicians. Further complicating the situation, one western official told CNN there is a strong possibility of the Beatles being stripped of their UK citizenship.
That would effectively leave them stateless, held by an unrecognized state controlled by Syrian Kurds in northern Syria. France's foreign minister has indicated that its citizens should be tried in northern Syria. But the Syrian Kurds are unlikely to have the capacity to try hundreds of ISIS members. So then what? Indefinite detention, or execution, by an unrecognized state? Would that mollify Europe's moral jitters here?
There is one possible solution. When US forces detain someone on the anti-ISIS battlefield, they sometimes hand them over to Iraqi security forces. Iraq is the only functional state where ISIS has territory, and with which the US has a diplomatic relationship. They have also been trying those ISIS fighters caught in their territory quickly and putting many on death row. Many detainees may have been to Iraq as well as Syria, perhaps putting their conduct under Iraqi jurisdiction.
In the meantime, the legal complications are mounting -- as is the number of detainees -- in a detention facility that's far from ideal. It is extraordinary that after nearly 17 years since the September 11 attacks, and after four years combating ISIS at home and abroad, Europe's capitals still stumble when working out what to do with this latest variation of defeated extremist.