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

British Isis fighters known as 'the Beatles' captured in Syria Read more

On the one hand there would be satisfaction that Alexanda Kotey, 34, and El Shafee Elsheikh, 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 Mohammed 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, Nicolas Henin, a Frenchman, on the BBC this morning, brought the whole horror back.

Play Video 2:14 'Quest for justice': former Isis hostage on capture of ‘Beatles’ – video

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 Elsheikh 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.

If they end up in American hands, what are their chances of any trial?

The answers being sought could include the proportion of foreigners among those who were fighting, and may perhaps still be fighting, for Islamic State. Now that Isis 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 Isis, 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 Elsheikh. It seems possible that either or both may have been stripped 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? Donald Trump recently signed an order to keep the Guantánamo 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 jihadists of their citizenship, where possible. Such are the complexities and the grey areas when war, non-state combatants and the law all become intertwined.

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