Nero Saraiva, a Portuguese citizen who became a permanent resident of the UK, is believed by officials to be one of "the most high value suspected ISIS detainees" anywhere in the world.
Saraiva was held by Kurdish forces in Syria following his arrest last year during the battle for Baghouz. But during the past few weeks he was secretly brought across the border to Iraq.
He could face trial there, or be extradited to Portugal.
Prosecutors in Lisbon believe he was behind a recruitment network of Jihadists that helped to find young men from Portugal and the UK and deliver them to the battlefield in Syria.
Saraiva is among eight men charged by Portuguese authorities for helping to recruit fighters for so-called Islamic State.
He is also suspected of having links to the abduction of British journalist John Cantlie, who was kidnapped in 2012.
Mr Cantlie’s whereabouts remains unknown.
Rohit Kachroo speaking in December 2019 explains how the recruitment process worked:
Saraiva travelled to Syria in 2012 where he is alleged to have continued organising a recruitment operation using friends who stayed behind in Europe and money raised in London through fraud.
He and his team advised recruits on which flights to book to get to Syria, the appropriate clothes to wear when travelling and even arranged for intermediaries to meet them once they arrived, according to police files seen by ITV News.
The network targeted young men in Lisbon and London, including one "petty criminal" who was arrested during the riots in 2011 - according to an investigation carried out by ITV News in conjunction with Portuguese magazine SÁBADO.
Cassimo Ture, another suspected member of the alleged cell, is living freely in east London.
When approached by ITV News this week he said he knew nothing about the allegations made by Portuguese prosecutors. "I don’t know Nero, I don’t know this dude," he said.
Taroughi Haydari from London, is one of "several militants" to have reached Syria during 2013 as a result of the alleged recruitment operation.
"You can send a message to him [Saraiva], that this is his fault," Haydari’s brother told ITV News earlier this week.
"He has taken my brother […] If he hadn't helped him to go to Syria then my brother would be alive.
"Tell him it is his responsibility.
"To the brothers, to their families, everything, he is responsible for it," he added.
Haydari’s brother, who does not want his full name to be revealed, said he welcomed the possibility that Saraiva might face justice in Iraq or Portugal.
"That's very good because my brother lost his life because of him. And if he hadn't gone [to Syria], then in four or five years afterwards he might have changed because at that age young people do whatever they want."
He said he recalled Taroughi Haydari falling under the influence of some "older friends", but assumed they were grooming him to become a petty criminal.
"Too much at nights he would go out with friends and do whatever bad things he was doing."
He recalls confronting his younger brother after he was arrested during the London riots: "I told him 'this is no good.'
"The last time, I told the police that this guy, my brother, needs to go for one year to jail and he will cool down, otherwise at 29 or 30 there will be chaos."
Saraiva, who used the alias Abu Yaqub al-Andalusi, was captured after escaping Islamic State group's so-called "Caliphate" before its territorial defeat last spring.
He was questioned by American interrogators while in Kurdish custody.
His next steps are unclear, but Portuguese authorities could ask for him to be returned to Lisbon to face trial there, rather than Iraq.
In 2018, ITV News revealed that Nero Saraiva attended a so-called Islamic State commander's "call to arms" in Syria during the early days of the terror group.
Foreign fighters were ordered to play a lead role in the fight for territory.