LONDON (Reuters) - If London-born convert Abu Rumaysah is confirmed as the front man in the latest Islamic State video, then he will be just the latest in a long line of militants to emerge from a banned group the authorities say breeds easy prey for jihadist recruiters.
The video, purportedly filmed in Syria, shows a masked man taunting British Prime Minister David Cameron in London-accented English, before five prisoners are killed.
Officials have not confirmed the man's identity. But British media such as the BBC, citing voice experts, say it sounds like past recordings of Abu Rumaysah, born Siddharta Dhar to a London Hindu family, who for many years gave speeches and interviews as a prominent figure in the group al-Muhajiroun.
His sister has told media the voice of the militant in the video sounds like her brother although she was not sure it was him.
The group was founded by Syrian-born Islamist cleric Omar Bakri in the late 1990s and called for Sharia law in Britain. It was banned under anti-terrorist laws in 2010.
Its leaders have maintained that they do not support violence, acting under a "covenant of security" that requires Muslims in non-Muslim countries to submit to the authorities.
The group gained media notoriety in Britain after the Sept. 11, 2001 attacks on the United States for issuing leaflets that referred to the hijackers as "the Magnificent 19".
It held regular meetings in community centers east London and would often stage demonstrations outside the prime minister's office in Downing Street calling for all governments to operate under a strict form of Islamic law.
Founder Bakri was banished from Britain in 2005 after four young British Muslims carried out suicide bomb attacks on London's transport network killing 52 passengers. He is now in jail in Lebanon.
While al-Muhajiroun itself was never found responsible for specific acts of violence, 23 of the 51 militant incidents or plots in Britain from the late 1990s until 2013 involved people that at one time or another had associated with the group, according to a study by Raffaelo Pantucci, director of International Security Studies at London-based think tank RUSI.
"If you look at all the people who do go off and try to set off bombs or do become involved in terrorist activity, you find that at some point in their past an awful lot of them have been involved in the group," Pantucci told Reuters.
Pantucci, author of "We Love Death As You Love Life", a study of Britain's suburban militants, said this did not mean that all al-Muhajiroun members gravitated toward violence.
In fact, many of those later convicted of terrorism offences had already stopped participating in the group, fed up with its lack of action and focus on protests, Pantucci said.
He cited the case of a leader of a cell, jailed in 2007 for plotting bomb attacks on targets across Britain, who had been caught on police wire taps ridiculing al-Mouhajiroun, even though he had been previously heavily involved with it.
"It's a very loud organization and the leaders in particular are very loud people so they attract attention," he said. Militants used the community around al-Muhajiroun "as a good kind of hunting ground to find potential recruits".
Adam Deen, a former al-Muhajiroun member until about 2003 who now works as a senior researcher for Quilliam, an anti-extremism think tank, said al-Muhajiroun gave angry, disenfranchised young Muslims a channel for their passion.
"What they do is they take these type of grievances and 'religousify' it and coach it in Islamist and Wahhabist terms and that person thinks they are serving God when they are committing an act of terrorism," Deen said.
Those connected to the group include Abu Hamza al-Masri, jailed for life in the United States last year for terrorism-related offences, and Michael Adebolajo, who killed a British soldier on a London street in 2013.
Last March, 19-year-old Muslim convert Brusthom Ziamani was jailed for 22 years after being found guilty of plotting to behead a soldier, after being arrested carrying a 12-inch (30-cm) knife and a hammer, wrapped in a black Islamic flag.
At his sentencing, Judge Timothy Pontius blamed al-Muhajiroun members for his radicalization: "I have little doubt that ... he fell under the malign influence of al-Muhajiroun fanatics who were considerably older, and had been immersed in extremist ideology far longer than him."
Since al-Muhajiroun was banned, members have operated under other labels, such as Islam4UK and Muslims Against Crusades, all of which were banned by the British government as other names for the same organization.
Deen, the former member, said the organization had become more radical over time.
"We were not in favor of terrorism. But those kind of restrictions are no longer there," he told Reuters. "Now things have changed and they have become more extreme in their views."
Editing by Guy Faulconbridge and Peter Graff