British Isis terrorist Jihadi John may be using a body double in a bid to fool military drones, according to new analysis of the video showing the murder of Syrian officers and pilots and the decapitated head of Peter Abdul-Rahman Kassig.

It also appears to suggest that Kassig fought his captors before he was beheaded.

In a horrifying video of the mass murder of Syrian soldiers and the American aid worker, Jihadi John appears with his face hidden, as usual - but analysts have discovered a single frame in which another man, visible for less than half a second, can been seen wearing a balaclava.

British Isis terrorist Jihadi John (right) may be using a body double in a bid to fool military drones, according to new analysis of the video showing the murder of Peter Abdul-Rahman Kassig (left)

They have speculated that he could be a decoy being used by Isis to lure drones away from Jihadi John.

Veryan Khan, editorial director with the Terrorism Research and Analysis Consortium (TRAC), which studied the video for over a week with Quilliam, the British anti-extremist think-tank, told The Telegraph: ‘It could be a body double or a decoy. I'm not trying to say I know the answer, but his shape and size are very similar.’

A spokesman for Quilliam told MailOnline that the other figure may also be someone other than Jihadi John, who is leading the slaughter.

The teams examining the footage also discovered a large bruise above Mr Kassig’s left eye, which may have resulted from a struggle with the fanatics before he was killed.

The fact that he was not shown reading a scripted message out is also evidence that he was defiant to the last.

It could be that he knew his execution was coming and denied his captors the satisfaction of repeating their words on camera.

Lasting 15 minutes and 53 seconds, the video – titled ‘Although the disbelievers dislike it’ – starts with a documentary-style history of Islamic State since the invasion of Iraq by western allies in 2003.

There are scenes of bombings, summary executions and graphic video footage of dead civilians lying in the streets. It also shows what appears to be a dead child lying on a bed covered in blood.

In the next clip, filmed in the desert, the self-declared murderer known as Jihadi John is seen leading a line of Syrian troops to their deaths.

A few paces behind him – and in the middle of a 16-strong line of Islamist jihadists said to be from over six countries - is the man believed to be Nasser

Unsettling: The video also shows the mass murder of Syrian military personnel by beheading led by Jihadi John at a desert location. Unlike the murder of Western hostages, the killing of the Syrians is shown in full

Bloodthirsty: The video shows in full graphic detail how the militants saw the heads off their victims

Muthana, a 20-year-old from Cardiff.

Like the other jihadists he is unmasked. John however keeps to his habit of showing only his eyes.

Muthana clutches the scruff of the neck of a Syrian soldier, whose hands are bound with plastic ties. But while the other militants have expressionless faces – this young bearded man looks menacingly into the distance. Taking the lead from John and those in the line before him, he brandishes a military knife taken from a box of weapons.

The chilling sound of a blade being drawn can be heard above the sound of Islamic music in the background. The prisoners are not wearing Guantanamo-style orange jumpsuits, as in previous videos, but are dressed in black trousers and shirts.

Humanitarian: Abdul-Rahman Kassig, the 26-year-old former Army Ranger, was committed to aid work

John, who has leather shoulder holsters for hand guns and a balaclava-style mask, takes centre stage.

His appearance would seem to confirm his seniority and status as ‘chief executioner’ of Islamic State. The man thought to be Muthana stands to his right. John delivers his message to America and Britain before an eerie silence during which the camera zooms in on the faces of the killers, who remain impassive throughout.

The scene then focuses on John – who begins to cut his prisoner’s neck – quelling suspicion that he himself did not actually perform the previous beheadings.

ISLAMIC STATE'S WESTERN VICTIMS James Foley - this 40-year-old American freelance photojournalist was documenting the Syrian civil war when he was snatched in November 2012 in the country's north west. His murder was shown in a video released in August.

Steven Sotloff - an Israeli-American journalist working for both TIME and The Jerusalem Post who was kidnapped in Aleppo, Syria, in 2013. The 31-year-old's apparent beheading was shown in a video released on September 2.

David Haines - the 44-year-old British humanitarian aid worker was taken hostage while working at a refugee camp in Idlib, Syria, near the Turkish border. His abduction only became public when he appeared in the Sotloff execution video. He was apparently killed on camera in a video released on September 13.

Alan Henning - this volunteer aide worker from Britain, 47, had been working with Islamic charities to deliver food and water to people affected by Syria's civil war. He was abducted in the city of Al-Dana when that area fell under the control of Islamic State fighters in December 2013. A video showing his apparent murder was released on October 3. Advertisement

Unlike the apparent murders of Western hostages, in which the camera cuts away at the moment their throats are cut, the killings of the soldiers are shown in full.

The camera zooms in on Jihadi John and his victim who, in his final agonising moments, closes his eyes. John then looks threateningly into the camera while still holding the knife. The next clip shows a knife slicing into the necks of Syrian soldiers followed by a stream of blood that turns the sand red.

The image after that shows the soldiers’ heads on top of their bent-over bodies.

The mass beheading is a new departure because previous mass executions have been undertaken using guns.

The fact that John’s accomplices revealed their identities suggests they have no intention of returning to their home countries. A caption refers to them as ‘hungry lions whose drink is blood and play is carnage’.

Family and friends of Mr Kassig were joined by politicians and prominent figures in the Muslim community at a memorial service Sunday.

The former Army Ranger, who was 26 at the time of his death, was remembered for his commitment to aid work at Butler University two days after his funeral services.

'We're not here because of how he passed,' said Hazem Bata, executive director Islamic Society of North America in Plainfield.

'We're because of how he lived. We're here because of the type of man he chose to be. We're here because of his selflessness.'

Kassig landed in the hands of ISIS extremists last year while delivering aid to Syrians caught in the crossfire of a ghastly civil war.

At the time he was working as part of Special Emergency Response and Assistance, or SERA, a relief organization he founded in 2012 and which suspended operations after his kidnapping.

His parents learned of his capture last year, though the information was not made public at the time.

When Kassig appeared in a beheading video of a British aid worker, his parents recorded a video pleading for his release to ISIS militants.

Kassig, who converted to Islam and changed his name from Peter during his time in captivity, inspired the Muslim community to come to his family's side.