However, neither American nor British officials could say for certain that the militant was the same man as Jamal Malik al-Harith, the man who had been held at Guantánamo.
The British government also said it could not confirm the Islamic State’s communiqué nor the reports in the British news media. “As all U.K. 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,” the Foreign and Commonwealth Office said in a statement.
American records show that a British man named Jamal Malik al-Harith was held at Guantánamo from 2002 to 2004. But no records suggest that anyone named Ronald Fiddler, reported to be the man’s birth name, or Abu Zakariya al-Britani, a nom de guerre, was held at Guantánamo.
According to Defense Department documents, Mr. Harith was born in 1966 in Manchester, England. He was detained by the Taliban while driving in Pakistan, and he was then held by the Taliban in Kandahar, Afghanistan, where he was detained by American forces in October 2001.
He was transferred to Guantánamo Bay in February 2002. That September, Maj. Gen. Michael E. Dunlavey, who was in charge of intelligence operations at Guantánamo, recommended that Mr. Harith be approved for release or transfer, based on an assessment that he “was not affiliated with Al Qaeda or a Taliban leader.”
Nonetheless, Pentagon officials had their doubts about the man. They noted that he had traveled extensively in the Middle East from 1992 to 1996, and that he had joined a Qaeda operative who went to Sudan in 1992 at the same time that Osama bin Laden was active there. The recommendation that he be released was overruled by Maj. Gen. Geoffrey D. Miller, the commander of the Guantánamo operation from November 2002 to March 2004.
However, the case of Mr. Harith and several other British citizens held at Guantánamo became divisive, and the government — then led by Prime Minister Tony Blair, who supported President George W. Bush’s decision to go to war with Iraq — helped secure the release of the Britons.
On Wednesday, Alex Carlile, a lawyer and member of the House of Lords who served as an independent reviewer of terrorism legislation in Britain from 2001 to 2011, explained the reasons for a financial settlement with Mr. Fiddler.
“The settlement of Fiddler’s claim is something that should not have been necessary,” he said in a phone interview. But if the government had not settled the lawsuit, he added, it would have had to disclose national security secrets.
Mr. Carlile said it was disturbing that the government did not appear to have knowledge or control of Mr. Fiddler’s movements after he returned to Britain.
“It’s a quandary,” he said. “It is absolutely plain and clear that he had significant radical associates.”
He added: “He was in this country, and he was able to leave and fight for ISIL, and that raises questions on border checks. That said, he had lain low, so attention was put on people who were more active.”
A number of Britons have gone off to fight for the Islamic State. Perhaps the most prominent was Mohammed Emwazi, nicknamed Jihadi John, who was killed in a November 2015 airstrike near Raqqa, Syria. Mr. Emwazi was shown in videos in late 2014 and early 2015 killing several American and other Western hostages.
According to the Islamic State communiqué, the fighter who went by al-Britani blew himself up in a car bomb on Monday during an attack on Iraqi Army and allied militias in Tal Kisum village, southwest of Mosul. It was not clear if or how many people were wounded or killed. In a related attack, an Iraqi militant set off a car bomber in an attack on a Russian-made tank in the area, according to the communiqué.
In a related attack, an Iraqi militant detonated a car bomb in an attack on a Russian-made tank in the area, according to the communiqué.