LONDON:(WN)-They called him Abu Zakariya al-Britani the surname means “the Briton” and they say he blew himself up on Monday in an attack at a village southwest of Mosul, Iraq. The claim, in a communiqué from the Islamic State, immediately revived fears about foreign fighters who have moved to Syria or Iraq to join the group’s ranks. But in Britain, it prompted even more troubling allegations.

Several British news organizations including the BBC, The Times of London and The Guardian reported Tuesday evening that the man was Jamal Malik al-Harith, a native of Manchester, England, who was captured in Afghanistan in 2001; detained by the United States in the military prison at Guantánamo Bay, Cuba, from 2002 to 2004; and released to Britain, where the government later awarded him 1 million pounds, about $1.25 million at current exchange rates, to settle a lawsuit.

The possibility that a former Guantánamo inmate had gone to Iraq to blow himself up stirred outrage in British tabloids. It also prompted an emphatic response on Wednesday from Tony Blair, a former prime minister of Britain whose decision to join President George W. Bush in invading Iraq in 2003 remains divisive. Mr. Blair said in a statement that he was being unfairly blamed for the Americans’ release of Mr. Harith to Britain, even though it was widely supported at the time.

“It is correct that Jamal al-Harith was released from Guantánamo Bay at the request of the British government in 2004,” Mr. Blair said. “This followed a massive media and parliamentary campaign, led by The Daily Mail, the very paper that is now supposedly so outraged at his release, and strongly supported by the then-Conservative opposition.”

Mr. Blair added that the compensation was agreed to in 2010 by the government of the Conservative prime minister at the time, David Cameron.

“The fact is that this was always a very difficult situation where any government would have to balance proper concern for civil liberties with desire to protect our security, and we were likely to be attacked whatever course we took,” Mr. Blair said. “The reason it did take a long time for their release was precisely the anxiety over their true affiliations.”

Mr. Blair did not say he knew for certain that Mr. Harith was the man who carried out the bombing claimed by the Islamic State, also known as ISIS or ISIL. His comments were limited to pushing back against criticisms of Mr. Harith’s release.

The Islamic State released a photograph of a militant it said was the bomber. The Times of London quoted a man named Leon Jameson as saying that the person in the photograph was his brother, Mr. Harith. “It is him, I can tell by his smile,” Mr. Jameson said. “If it is true, then I’ve lost a brother.”

However, neither American nor British officials could say for certain that the militant was Mr. Harith, who was also known as Ronald Fiddler. “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.

Maj. Ben Sakrisson, a spokesman for the Pentagon, said in a statement: “I can confirm that an individual named Jamal Malik al-Harith was detained in the Guantánamo Bay detention facility from February 2002 to March 2004, when he was released to the United Kingdom. However, we cannot confirm his death, as the occurrence of the same name does not necessarily equate to this being the same individual.”

According to Defense Department documents, Mr. Harith was born in 1966 in Manchester. 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 British government then led by Prime Minister Tony Blair, who supported President George W. Bush’s decision to go to war with Iraq helped secure their release. 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 the financial settlement with Mr. Harith.

He said the settlement “should not have been necessary,” but that if the government had not made the payment, 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. Harith’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 the Britani name blew himself up in a car bomb on Monday during an attack on the Iraqi Army and allied militias in the village of Tal Kisum, southwest of Mosul. It was not clear if others were wounded or killed. 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.