Authorities suspect that the man seen in the video released Sunday by the Islamic State group is well-known British radical Siddhartha Dhar, also known as Abu Rumaysah, according to reports. The news comes as U.K. investigates the video that features a new “Jihadi John” and a young boy, and shows the execution of five prisoners identified as British spies by the extremist group.

Dhar, who is believed to be the masked jihadi in the ISIS execution video, fled to Syria over a year ago after he and his mentor, radical London cleric Anjem Choudary, were arrested by Scotland Yard on charges of supporting terrorism, officials told ABC News. Dhar hails from Walthamstow in east London. He was a Hindu who converted to Islam and joined the radical group al-Muhajiroun.

Dhar's sister told the BBC that when she first heard the audio of the ISIS video she feared it was her brother, but was unsure after watching the video.

"I was in a state of shock," Konika Dhar told BBC. "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 video, five men are depicted kneeling in orange jumpsuits before masked gunmen. After being accused of spying for Britain, they each deliver a confession in Arabic. At the end of the video, all the five men are shot in the head.

“This is a message to David Cameron,” a masked jihadi militant says in the video. “How strange it is that a leader of a small island threatens us with a handful of planes,” the man says, mocking British military efforts to eliminate the extremist group in Iraq and Syria.

screenshot Prisoners are seen kneeling before their execution in an Islamic State group propaganda video released Sunday. Photo: YOUTUBE/SCREENSHOT

After the apparent killings, a young boy seen in the video wearing military-style clothing, reportedly points into the distance and talks about killing the "unbelievers."

The English-speaking boy is suspected to be the son of a south London woman who converted to Islam before heading to Syria, the Wall Street Journal reported, citing the woman’s father.

 “He’s under 5. He was acting under the influence of the IS leaders,” Henry Dare told U.K. media ITV Monday, adding that he believed the boy is the son of his daughter, Grace Dare.

However, Henry’s claim could not be independently verified, the Journal reported.

The masked man in the video also recalled the fighter dubbed "Jihadi John," a British Arab who appeared in apparent beheading videos released earlier by ISIS. U.S. officials have reported killing "Jihadi John," whose real name was Mohammed Emwazi, in a November drone strike.

On Monday, U.K. Prime Minister David Cameron responded to the ISIS execution video saying that the country will not be intimidated by the group’s terrorism.

“It’s desperate stuff, from an organization that really does do the most utterly, despicable and ghastly acts, and people can see that again today,” Cameron reportedly said. “But this is an organization that’s losing territory, it’s losing ground. It’s, I think, increasingly losing anybody’s sympathy, and this again shows what an appalling organization we’re up against.”