The sister of jihadi Abu Rumaysah says she will “kill him myself” if he is confirmed as the masked executioner in a new Islamic State (IS) video in which five hostages are killed.
Konika Dhar, from north London, said she does not believe her brother, formerly known as Siddhartha Dhar, is the man in the video, but she conceded the voice sounded "a bit like him".
She told the Press Association: "I don't think it is, personally, to be honest."
She went on: "I examined the features – one eye is smaller than the other, my brother has symmetrical eyes. The eyebrows are bushy and this guy is taller, my brother is shorter and he has got broader shoulders, but he has got stooped shoulders so I don't think it is.
"I think the voice sounds a bit like him, but it could be someone else, but he could have helped with a voiceover, but I have no idea."
She said she has not had contact with her brother in more than a year, saying she “doesn’t even know who he is” any more.
She also pleaded with the UK authorities to provide an alternative for those who have been radicalised and travelled abroad, to help them return home.
Ms Dhar said: “If it is him, bloody hell am I shocked? I am going to kill him myself. He is going to come back and I am going to kill him if he has done this.
“I can’t believe it. This is just so shocking for me. I don’t know what the authorities are doing to confirm the identity, but I need to know if it is.”
In the video a masked man, speaking with a British accent, calls David Cameron an "imbecile" and mocks the RAF's bombing campaign against IS which was recently extended to Syria.
Five men accused of spying for the British security services, dressed in the orange jumpsuits reminiscent of earlier IS victims, are made to kneel and then shot in the back of the head.
A little boy appears at the end of the 10-minute video after the executions. Dressed in military fatigues and wearing a black headband, he warns that Isis will "kill the kuffar", which means non-believers.
Professor Jane Setter, professor of phonetics at the University of Reading, said the executioner used an accent known as Multicultural London English (MLE) which is common among younger people in London.
But, she added, he uses fewer aspects of the MLE accent than Emwazi, which led her to suspect he either grew up in the UK, arrived at a younger age then Emwazi (who came to Britain aged six) or has spoken English for longer.
She said: "This is similar to the accent of Mohammed Emwazi... Among other features of his speech, he pronounces a 'th' sound as a 'd', for example saying 'dis' rather than 'this', and his vowels in words like 'planes' and 'today' sound less like an 'ay' and more like an 'e' sound.
"However, unlike Jihadi John, he has a clear 'L' sound at the end of words like 'people' and 'imbecile' - a feature which is not that common in British accents, but is heard in Welsh and in Indian English, for example. He also has a rather strongly affricated 't' as the end of some words like 'state' - almost an 's' after the final 't' - but not in all cases."
Prof Setter said the executioner's speech was "closer in many ways" to Southern Standard British English and "much less syllable-timed than Emwazi as far as speech rhythm is concerned".
She added: "I also note that, unlike the Jihadi John video, the movements of the speaker's jaw are much more discernible. This leads me to believe he is most likely speaking live and not that the masked face has been overdubbed later.
"Of course, detailed forensic analysis would be needed to verify these conclusions."