The prime suspect in the latest propaganda video of the self procalimed Islamic State is thought to be Siddharth Dhar, a British man, the BBC has reported.
Mr Dhar was a Hindu who converted to Islam and joined the radical group al-Muhajiroun.
Mr Dhar is the focus for the security service investigating the video, which purports to show the killing of five men who IS says were spying for the UK.
"A lot of people think it is him," a source told the BBC, although there has been no official confirmation.
Mr Dhar - also known as Abu Rumaysah - was arrested in 2014 but jumped bail.
The former businessman and father-of-four, from Walthamstow in east London, then made his way to Syria. He had been arrested on suspicion of encouraging terrorism.
Earlier, one of his associates told the BBC he had "no doubt" that the voice on the video was that of Mr Dhar.
Mr Dhar's sister told the BBC that when she first heard the audio of the video she feared it was her brother, although having watched a clip she was now not so sure.
"I was in a state of shock," said Konika Dhar.
"I believed the audio to resemble, from what I remember, the voice of my brother but having viewed the short clip in detail, I wasn't entirely convinced which put me at ease".
In the latest video by the extremist group - which has not been independently verified - the masked man, who is holding a gun, mocks Prime Minister David Cameron for daring to "challenge the might" of the extremist group.
He goes on to say: "We will continue to wage jihad, break borders and one day invade your land where we will rule by the Sharia."
Mr Cameron said the video was "desperate stuff" from a group that was "losing territory".
