The British security services have been sifting through social media in a bid to identify the British-sounding man who appears in the latest propaganda killings video by so-called Islamic State.
ITV News Security Editor Rohit Kachroo said the security services will have a "fairly good idea" as to the leading candidates among the several hundred British jihadists who have signed up for IS.
"There are a few names doing the rounds", Kachroo told ITV News at Ten. "One of them is Siddhartha Dhar."
He said Dhar's sister and his mentor have told ITV News he is not the masked man who appears in the latest video to goad Prime Minister David Cameron as a "slave to the White House" before five hostages are executed.
But a documentary film-maker who interviewed Dhar on camera in 2014 told ITV's Good Morning Britain he believes he is the man in the video.
"Judging from the voice it sounds very much like him," Mojtaba Ali Masood said. "I recognised the voice and I thought 'wow, this is the man that I interviewed'."
Mr Masood said Dhar was a likely candidate to appear on camera issuing terror threats in the wake of the British involvement in the airstrikes on IS targets in Iraq and Syria.
"People like him enjoy the media presence," he said. "He's a charismatic recruiter that Isis use to exploit their propaganda message."
Dhar, who is also known as Abu Rumaysah, fled to Syria in 2014 while on bail after being arrested on suspicion of encouraging terrorism.
His sister, Konika Dhar, agreed the voice in the latest IS video sounded "a bit like him" but said she was not convinced it was her brother, who she said she had last had contact with over a year ago.
Ms Dhar, a law student from north London, told the Press Association she will "kill him" if he is confirmed as the man in the video but said: "I don't think it is, personally, to be honest."
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.
Ms Dhar said she was certain the young boy in the footage was not her brother's son.
"I can 100% say that is definitely not my nephew," she said. "So I was more or less relieved, in a sense, and I hope that it's not (Siddhartha Dhar)."
London grandfather Henry Dare told ITV News he believes the boy featured in the latest IS video is his grandson.
Mr Dare, whose daughter is the known British jihadist Khadjiha Dare, said he had not seen the boy for more than three years, ever since his daughter took him with her to Syria.