New ‘Jihadi John’ in ISIS Video May Be British Radical, Officials Say The new masked executioner may be well-known radical Siddhartha Dhar.

 -- ISIS’ new “Jihadi John” may be a well-known British radical, authorities tell ABC News.

Authorities are investigating whether it is British radical Siddhartha Dhar -- also known as Abu Rumaysah -- who fled to Syria a year and a half ago after he and his mentor, radical London cleric Anjem Choudary, were arrested by Scotland Yard on charges of supporting terrorism.

In the video, released through ISIS channels on Jan. 3, a group of ISIS militants led by a masked man with a British accent execute five supposed British spies. The leader of the group, wearing camouflage and a balaclava, calls British Prime Minister David Cameron “an imbecile” and promises retribution against the United Kingdom’s campaign in Syria.

Officials are trying to make a final determination as to who the masked man is, but believe it may be Dhar.

ABC News correspondent Lama Hasan interviewed Dhar in 2014, as his group took part in a patrol to encourage and enforce Islamic Sharia law in London.

“We are here to call for a complete and utter regime change in Britain,” he told ABC News at the time.

In the interview, Dhar continued to urge young Muslim men to push for Sharia law, calling jihad “a noble aim” and emphasizing the Muslim identity over occupation or nationality.

“We’re Muslim first, Muslim second, Muslim last,” he proclaimed.

Dhar was a Hindu living in Walthamstow, a suburb of East London. He later converted to Islam and joined the radical group Al Muhajiroun, at which point he began using the name Abu Rumaysah.

A former associate from Al Muhajiroun told the BBC today he “has no doubt” that the masked executioner is his former associate Dhar, saying “the voice is unmistakable.”

Dhar’s sister initially told the BBC she thought she recognized her brother’s voice but could not be certain.

“I was just in a bit of, sort of state of shock, because… I believe the audio to resemble from what I remember the voice of my brother, but I think sort of having viewed the short clip in detail, I wasn't entirely convinced, which sort of put me at ease,” she said.