One of the British ISIS fighters known as "The Beatles", who was moved to US custody after Turkey launched its assault on Kurdish-held territory in Syria, said just before his transfer that he wanted to return to the UK.
President Donald Trump stunned many by tweeting on Wednesday that two of the men were now in a "secure location controlled by the US".
The four extremists, nicknamed because of their British accents, were said to be notorious even among fellow ISIS members for sadistic torture methods and execution videos.
Their list of beheading victims included US reporter James Foley and UK aid worker David Haines.
They were among other Europeans who joined ISIS as the terror network began capturing large areas of Syrian and Iraqi territory in 2014.
The US State Department identified one of the four as El Shafee Elsheikh, a Sudanese-born Briton whose UK citizenship has since been revoked.
Mr Elsheikh, while he was still being held by Kurdish fighters in a northern Syrian prison camp last week, told a team from ITV television that he felt unsafe and wanted to stand trial in Britain.
"It's very unstable," he said of the security situation around the Kurdish camp.
"If the UK wants to put me on trial then I will defend myself with what I can. I will admit to what I admit to, and defend myself on what I defend myself on."
Mr Elsheikh said he had "never committed a crime in the United States".
The State Department said he worked as a mechanic in London before joining ISIS. He was captured by the mostly Kurdish Syrian Defence Forces in January 2018.
Mr Trump is demanding that European countries take back their own fighters.
Overstretched Kurdish forces now have the fight against Turkish soldiers to focus on, instead of keeping watch over thousands of ISIS captives.
But Britain remains uncertain about how to handle the potential return of hardened warriors who denounced western culture and fought for its demise.
Mr Haines' daughter Beth said she heard of the two men's transfer to US custody with relief.
Her father was executed by "Beatles" member known in the British press as "Jihadi John", who was killed in a US drone strike.
"Justice is what we are waiting for, that's the end of the story," Bethany Haines told ITV. "It is hugely important and it is such a relief that the chances of them escaping are very much lessened now.
"But if he thinks that for one minute he's going to have an easy time of it in that courtroom ... when he sees me, he's got another thing coming."