A trail of videos also offers some insight into Dhar's world view.
In one posted on his YouTube website, titled "The beauty of the Shariah and the Caliphate", he said: "I've grown up in the West, I've lived in the United Kingdom all my life, I've seen what a democracy has to offer and quite frankly it's quite oppressive."
His sister Konika told British media the man in the video sounded just like her brother, echoing the views of others who knew him well. However, neither she nor security experts said they could be certain it was him.
British intelligence officials are analyzing the film and a security source said they had not yet reached a conclusion as to the man's identity.
British officials say up to 800 Britons have traveled to Iraq and Syria, some to join Islamic State, and around half had returned home. About 70 were believed to have been killed.
In the 10-minute video released on Sunday, the masked man speaking with a clear British accent threatens British Prime Minister David Cameron before shooting one of the alleged spies in the head.
The footage was reminiscent of the gruesome beheadings shown in videos featuring Londoner Mohammed Emwazi, who gained worldwide notoriety as "Jihadi John".
Robb Leech, who got to know Dhar while making a documentary about his own stepbrother who was jailed for terrorism offences in 2013, said the militant in the video looked and sounded like Dhar. He said Dhar had never struck him as violent, and had always been friendly.
British newspapers said he used to sell children's inflatable bouncers, known as bouncy castles in Britain, and was a fan of Arsenal soccer club and the rock group Nirvana.
"The last time I saw him it was about two months before he left and joined so-called Islamic State and we were laughing. It was very good natured," Leech told BBC radio.