A British IS fighter who died in a suicide bomb attack on Iraqi forces in Mosul is a former Guantanamo Bay detainee, the BBC understands.
The self-styled Islamic State group said two days ago that Abu-Zakariya al-Britani detonated a car bomb at an Iraqi army base in Tal Gaysum, south-west of Mosul.
He is believed to have been originally known as Ronald Fiddler.
Fiddler, 50 and from Manchester, was sent to Guantanamo Bay in 2002.
IS has now published a photograph of Fiddler, who was also known as Jamal al-Harith before taking the nom-de-guerre Abu-Zakariya al-Britani.
Al-Harith was handed over to American forces in Afghanistan in early 2002.
He was taken to Guantanamo where US interrogators found he provided useful information to them about the Taliban's methods, and he was released after two years.
The Daily Mail reported Fiddler received a million pounds in compensation from the government when he came back to the UK.
The BBC has seen IS registration papers signed by Fiddler in April 2014 when he crossed into Syria from Turkey.
He volunteered to be a fighter, saying his knowledge of Islam was basic.
His wife told the Daily Mirror that she and their five children went to Syria try to persuade him to come back, but failed, and they ended up having to flee for their lives from IS territory.
Afzal Ashraf, a former counter-terrorism adviser to the United States in Iraq, told the BBC this incident demonstrated that "some of the people in Guantanamo Bay were up to no good".
"It also throws into question some of the organisations that were supporting him, that brought him back [from Guantanamo].
"Some of them were associated a little too closely with this radical ideology, and they use the legal system - freedoms of speech, the importance of the rule of law - in order to subvert some of our systems in the UK and elsewhere."
BBC security correspondent Frank Gardner says the "pipeline" of British and European jihadists who once crossed the Turkish border into Syria, into IS-controlled territory, has now "pretty much dried up".
He added: "Guantanamo Bay was incredibly bad in the ideological fight against extremism. It makes it very difficult for America, Britain and other countries whose nationals are there to maintain any kind of moral high ground, because they [detainees] were imprisoned without trial.
"And what do you do about the estimated 400 British jihadists who are still out there? If they don't die on the battlefield, are they going to try and come back? How do you monitor them? Do you believe the people who say 'I've turned my back on all of that'.
"Most people will probably want to do exactly that. Some, though, may have other ideas."
According to figures published by the UK government last year, about 850 people regarded as a national security concern have gone to become fighters in the Middle East.
Of those, just under half have returned to the UK and approximately 15% are dead.
A Foreign Office spokeswoman said: "The UK has advised for some time against all travel to Syria, and against all travel to large parts of Iraq.
"As all UK consular services are suspended in Syria and greatly limited in Iraq, it is extremely difficult to confirm the whereabouts and status of British nationals in these areas."