Video report by ITV News Security Editor Rohit Kachroo
ITV News has exclusively obtained the first image of Alexanda Kotey,the suspected member of Jihadi John's British execution gang, in detention.
The bearded Kotey is seen in a tatty grey t-shirt after being captured in Syria in January, trying to smuggle himself into Turkey.
Sources have told ITV News he was attempting to eventually get back to Europe when he was held on January 24 alongside his friend El Shafee Elsheikh.
Redur Khalil, a spokesman for the Syrian Democratic Forces, confirmed to ITV News the detained man pictured was Kotey.
He said he was a "member of the daesh terrorist group" who was arrested in January by "the anti-terrorist forces of the Syrian SDF in the Raqqa countryside while he was trying to flee to Turkey".
He detailed how Kotey, from west London, had attempted to get to Afghanistan more than a decade ago.
"He was selling drugs in London, he was arrested many times there," he said. "In 2005 with the help of a British man of Afghan origin he tried to go to Afghanistan. He went first to Barcelona. From Barcelona he flew to Turkey but the authorities sent him back because he had a jihadi record.
"In 2012 he went back to Turkey through Greece and entered Turkey even though the Turkish intelligence had his jihadi record. He gained two months' residence in Turkey and then he was allowed to go to Syria and he entered Syrian soil through the border crossing at Bab al Hawa."
Once he arrived he was allegedly put to work by a jihadist group.
"He became the emir in charge of foreign prisoners," he said. "While he was in charge of that prison he met the foreign prisoners, the US journalist James Foley and the British journalist John Cantlie."
According to Mr Khalil, Kotey soon joined so-called Islamic State.
"He moved to Raqqa, where he was also in charge of foreign prisoners until 2014. He then became the emir of the foreign fighters training camp.
"(Kotey later) became the translator of the media section in ISIS until the end when he planned to flee to Turkey after the SDF surrounded the ISIS areas. Now he is in the custody of the SDF and under interrogation."
Kotey and Elsheikh are said to be one half of the infamous four-man extremist group dubbed "The Beatles" because of their English accents.
The other members of the group were ringleader Mohammed Emwazi - popularly known as "Jihadi John" - and Aine Davis.
Emwazi, who appeared in horrific videos showing the beheading of British and American hostages, was killed in a British drone strike in 2015.
Davis is in custody in Turkey where he was convicted on terrorism charges.
All four members of the group had lived and grown up in west London.
Former child refugee Elsheikh was a mechanic from White City, and Kotey was from Paddington.
El Shafee's brother Mahmoud died fighting for so-called Islamic State in Iraq in 2015 after following his brother to the region.
On Friday, his parents refused to comment on their son's detention.
"This is not the right time for us," El Shafee's father Rashid said. "This is a mum who lost two. Her son now is in the hands of people, we don’t know what is going on."
Bethany Haines, whose father David Haines was beheaded by Emwazi in 2014, welcomed reports of the capture of Kotey and Elsheikh.
"No punishment is enough for these barbarians and in my opinion they should be sentenced to a slow, painful death," Ms Haines said.
Emwazi is also believed to have killed another British captive, Alan Henning, and the American journalists James Foley and Steven Sotloff, while the US government has attributed 27 beheadings to the group.
Foley's mother Diane told ITV News that Kotey and Elsheikh's detention was "a positive development".
“My hope is they will be brought to trial and face the consequences of their horrific actions," she said.
"I feel it's very important they are detained and held because they have caused so much pain to so many good people in the world.
"We cannot bring Jim back but we can find justice and hopefully stop this horror that they have inflicted on the world."
Last year the US State Department named Kotey a Specially Designated Global Terrorist, claiming that he "likely engaged in the group's executions and exceptionally cruel torture methods, including electronic shock and waterboarding".
The duo could be prosecuted in Syria but it is believed to be more likely they will go the United States and possibly the Guantanamo Bay detention camp.