A senior ISIS fighter was interrogated by western intelligence agents in July after being captured by Kurdish forces near the area where the terrorist group lost the last of its Syrian territory.
Officials believe Nero Saraiva, a Portuguese man who lived in London for 10 years before joining the terrorist group, could have information about the inner workings of the organisation, including the whereabouts of journalist John Cantlie.
The British writer and photographer was kidnapped in Syria with American James Foley in November 2012 and remains a hostage. Foley was killed by ISIS in 2014 and his murder broadcast online.
"He [Saraiva] was one of the first fighters to arrive in Syria and lasted the duration," an official told The Times. "He's potentially a goldmine of information."
Saraiva, 33, left his home in Walthamstow, north-east London in 2012, following in the footsteps of those he had helped to join the terrorist group. He left behind his wife and child and is rumoured to have taken five more wives and fathered up to 10 children.
Security services believe Saraiva may have been close to a small band of British ISIS members nicknamed The Beatles, serving as right-hand man to ISIS executioner, Mohammed Emwazi.
Saraiva demonstrated knowledge of ISIS plans before they were carried out, indicating his senior role within the group.
In July 2014, he tweeted a “message to America” about an ISIS movie. Just 39 days later, the group released the video of Foley’s murder.
Mr Cantlie was kidnapped alongside Foley in 2012 and forced to front a series of ISIS propaganda videos. He has not been seen since 2016, but in February the UK’s security minister at the time claimed Mr Cantlie was alive and still in the hands of the group.
Last week, Portuguese magazine Sabado reported Saraiva's capture in Baghouz this year. The report quoted Angela Barreto, a female ISIS member currently in Al Hol camp where thousands of ISIS families are being held. Ms Barreto said Saraiva was unable to walk, having been injured during the fighting.