Abu-Zakariya al-Britani was born in Manchester as Ronald Fiddler and then took on the name Jamal al-Harith once he converted to Islamfrom Christianity. He was detained at the Guantanamo between 2002 and 2004.
According to British media reports, he was reportedly paid 1 million pounds in compensation by the UK government after his release in 2004 from the US-operated Camp X-Ray military prison in Guantanamo.
"It's obvious that collectively the authorities - and obviously I have some personal responsibility there - we failed to be aware of what Fiddler was up to," Arthur Snell, the former head of the UK government counter-terrorism strategy Prevent, told BBC Radio.
"Clearly there was a high-profile figure, there was no mystery about this man, he was someone known to the authorities," he said.
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Fiddler was taken to Guantanamo after being found in prison in Afghanistan early in 2002, where he had been placed after being intercepted by the Taliban, who believed him to be a British spy.
It had emerged over the weekend that Al-Britani, a common name used by fighters who have come from Britain, was the only foreigner among five suicide missions reported by ISIS propaganda outlets since the start of the Iraqi army advance on the city of Mosul.
He was one of an estimated 850 Britons who are believed to have travelled to Syria and Iraq to fight for ISIS or other terror organisations. An estimated 15 per cent of them have been killed.
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