It is not hard to imagine the mixed feelings in Whitehall when British ministers learned that two of the most notorious British terrorists had apparently been captured in Syria by US-backed Kurdish fighters.

On the one hand there would be satisfaction that Alexanda Kotey , 34, and Al Shafee Al Shaikh, 29 — the two members of the quartet dubbed “The Beatles” who were still at large — were now out of circulation and could, potentially, face justice. Together with Mohammad Emwazi — aka Jihadi John — and Aine Davis , the two are implicated in some of the most flagrant and barbaric acts of the conflict, including the beheading of up to 27 foreign hostages and the torture of many more. The quiet reminiscences of one of their former hostages, the Frenchman, Nicolas Henin , on the BBC, brought the whole horror back.

On the other hand, however, their capture alive and their apparent presence now in the hands of an American-backed militia raises all sorts of questions about what should happen next. These are not questions that have arisen before. Emwazi was killed in a drone strike in Raqqa two years ago — and while the legality of such drone strikes can be contested, his was a classic case of “live by the sword, die by the sword”. Davis is in prison in Turkey after being convicted of terrorism crimes there.

How to handle Kotey and Al Shaikh is far more complicated. Initial reports suggest that the pair were seized by US-backed Syrian rebel forces in mid-January, then at some stage made available for interrogation by US forces. It would appear that they have been debriefed over several weeks, before news of their capture reached the wider world. It has not been said that the two refused to answer questions, so it is reasonable to assume they have been talking, revealing — perhaps — precious intelligence.

The answers being sought could include the proportion of foreigners among those who were fighting and may perhaps still be fighting for Daesh [the self-proclaimed Islamic State of Iraq and the Levant]. Now that Daesh has lost most of its territory, what are those fighters doing in retreat? Who is looking for new wars to fight; who is trying to get home? With an estimated 850 Britons believed to have joined the war in Syria and Iraq on the side of Daesh, and considerable disagreement on the part of the authorities about whether they should be arrested or reintegrated on their return, any insight is valued.

The question of repatriation, however, may not arise with Kotey and Al Shaikh. It seems possible that either or both may have been s tripped of their UK citizenship under Home Office provisions. And if this is so, what claim — if any —would the British authorities have on them? Would this allow the US authorities — or perhaps their Syrian Kurdish captors — to deal with them how they choose?

If they are still with Syrian rebel forces, then they could, at best, be held as prisoners of war. If they end up in American hands, what are their chances of any trial? US President Donald Trump recently signed an order to keep the Guantanamo Bay detention centre open — Barack Obama had failed in his promise to close it — and this would be a probable destination.

The prospect then would be for a repeat of the malign cycle of extrajudicial measures introduced by the US after 9/11 that has so damaged its international reputation for upholding law and justice, and tainted the UK by association. A far preferable course would be for them to face justice in court, best of all in the UK. But this would undermine the government’s policy of stripping some terrorists of their citizenship , where possible. Such are the complexities and the grey areas when war, non-state combatants and the law all become intertwined.

— Guardian News & Media Ltd

Mary Dejevsky is a writer and broadcaster, and a former foreign correspondent in Moscow, Paris and Washington