Analysts probe earlier video

American journalist James Foley. Photo / AP

Security agencies hunting for the killer of James Foley, the American journalist, are investigating video footage of two British jihadists taking part in the execution of a soldier captured during fighting in Syria.

British and US counterterrorism analysts believe one of the Britons bears a resemblance to the jihadist who beheaded Foley. If they can prove it is the same man, it would represent a breakthrough in the inquiry.

The footage, which was posted at the start of May on Instagram, shows a soldier loyal to President Bashar al-Assad crouching in a field in Syria, then being shot in the back of the head by a man armed with a pistol. A man with an automatic rifle then shoots the body.

MI5 is understood to be paying special attention to an English-speaking man standing beside the British jihadist who is alleged to have carried out the execution.

He is wearing a similar black robe and mask to that worn by the jihadist who beheaded Foley, and is of similar build and height. He is also - like Foley's executioner - apparently left-handed.

The May video represents the first clear-cut evidence of Britons being implicated in war crimes committed in the Syrian conflict. Researchers at the International Centre for the Study of Radicalisation, based at King's College, London, believe the men in the footage are both from Britain and part of a militant group called Rayat al Tawheed (Banner of God), which is closely linked to Isis. There are reports that the group has been involved in beheadings.

Rayat al Tawheed claim to have four British fighters with them, two of whom feature prominently in their videos. They both wear distinctive balaclavas and black watches.

The fighter resembling Foley's killer has the same hectoring style of delivery as that used in the recent Isis video. He pronounces on the evils of the West and beseeches British Muslims to donate money to fund their terrorism. A security source said: "We are considering all evidence from the internet to establish who killed James Foley. We can't afford to rule anything out. But at the same time nothing can be taken at face value."

In its analysis of the May video, the study centre said that the man who actually uses his AK-47 to fire shots into the body of the prisoner features prominently in several other Rayat al Tawheed videos.

But on its Twitter account Rayat al Tawheed denied knowing the identity of Foley's killer. It stated: "To all the journalists, I don't know who the masked man is that took off the head of Foley. Thank you for understanding." The centre found the footage of the soldier's execution, which took place in Raqqa in northern Syria, on the Instagram account of a man thought to be from London.

In the hunt for Foley's killer, security services have reportedly narrowed the field of suspects although a definitive identification has not yet been made.