The chilling masked figure of the fanatical Briton known as Jihadi John towered over Alan Henning in all too familiar style today as the taxi driver became the fourth hostage that he seemingly beheads.

British and US intelligence agents have identified the cold blooded killer and Islamic State propagandist in chief but his name has not been released as security investigators monitor those he is known to have been associated with.

Again last night the video had been carefully filmed in a bid to confuse those looking at its every aspect for clues but a picture has emerged of a monster who enjoys murder, humiliation and torture.

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Chilling: The masked figure of the fanatical Briton known as Jihadi John towered over Alan Henning in all too familiar style yesterday today as the taxi driver became the fourth hostage that he seemingly beheads

Security experts believe the black-robed killer ranting on the video has a distinct English accent, rather than another British person dubbing over the footage and comes from south east London.

The jihadist wears a black mask, concealing all but his eyes, making it difficult to be sure the voice on the video belongs to him. But audio experts have claimed they are ‘99 per cent’ certain the words were spoken by the man seen on camera.

So far the Briton has been seen murdering UK aid worker David Haines, and US journalists James Foley and Steven Sotloff but they also believe he has overseen the brutal, callous torture of hostages in his charge.

One of 500 Britons fighting with IS, he is now one of the most wanted men in the world with Special Forces hunting his headquarters in the group’s headquarters of Raqqa, northern Syria.

Jihadi John is understood to have become radicalised almost overnight and left Britain more than a year ago, where he quickly became a hardened fighter.

British and US signals specialists in Baghdad and the Kurdish city of Irbil have been working with GCHQ to pinpoint where he is now. Aircraft fitted with sophisticated equipment, which can eavesdrop on conversations taking place tens of thousands of feet below, are listening to see if they can pick up a voice match.

Details: All three previous murder videos have been broken down piece by piece and studied to provide clues on where they were filmed. Above, Jihadi John pictured before he beheaded Steven Sotloff

All three previous murder videos have been broken down piece by piece and studied to provide clues on where they were filmed.

Hostages freed earlier this year said the killer was one of three UK-born militants who held captives in Raqqa – who they nicknamed John, Paul and Ringo after members of The Beatles.

Jihadi John is said to be the most intelligent of the trio. He has taken responsibility for communicating with the outside world via negotiations with the prisoners’ families and beheading videos.

He also seems to have been in charge of security, reportedly ordering captives to be moved at least ten times to foil attempts to rescue them.

Paul and Ringo are thought to have been more junior, but together with Jihadi John they carried out the most sadistic beatings.

There were also French-speaking extremists among the guards, including at least one Belgian. But the British jihadists were the most feared because of their gruesome ‘taste for the macabre’, according to a freed French hostage.

The violence included beatings, use of Taser guns, and waterboarding – a form of torture where water is poured over a cloth blocking the nose and mouth.

They forced some of the Western prisoners to take part in mock killings – US journalist Mr Foley was forced to stand in a crucifix pose. The captives were also mostly kept underground in the dark, often shackled to one another.

The first beheading video sent shockwaves across Britain as the killer spoke with a distinctive London twang and English appears to be his first language.

A number of Britons were named as suspects, including a rapper from West London whose father faces terror charges in the US, a Birmingham student and a man from Portsmouth but security officials say that Jihadi John has never been named or identified in public.

Linguistic experts believe the way he pronounced certain words, such as Iraqi place names, suggest he has a foreign language background – indicating his parents or grandparents may have come from the Middle East. He appears to have a skin tone consistent with African, Middle Eastern or South Asian descent.

Academics claim Jihadi John has a multicultural East London accent, a kind of ‘new cockney’. The voice patterns will have been studied in painstaking detail by security chiefs desperate to glean his identity and match it with snippets of conversation picked up in Syria.

Threat:The jihadist wears a black mask, concealing all but his eyes, making it difficult to be sure the voice on the video belongs to him. At the end of the video released last night war veteran Peter Kassig is threatened

Aircraft fitted with sophisticated equipment, which can ‘eavesdrop’ on conversations taking place tens of thousands of feet below, are listening to see if they can pick up a voice match – potentially leading them to the location of the terrorist.

However, his voice in the latest beheading video appears to be distorted, suggesting IS are trying to disguise it.

Experts believe the killer is aged under 30. He is tall and slender, and appears to be left-handed.

Experts believe the killer is behind the final e-mail sent to the family of James Foley.

First appearance: The beheading of U.S. journalist James Foley, above, on August 19 marked the first in a string of grisly killings by the Islamic State fighter who has since become known as Jihadi John

In the third beheading video his feet are not visible – different from footage of him murdering the two Americans when he wears modern desert boots.

Ex-captives have described Jihadi John as intelligent, educated and a devout believer in radical Islamic teachings.

He is thought to be behind a final email sent to James Foley’s family, dated August 12 – seven days before the journalist was beheaded.

In a hate-filled message, the jihadis vowed to kill Mr Foley saying: ‘You [America] and your citizens will pay the price of your bombings! The first of which being the blood of the American citizen James Foley! He will be executed as a direct result of your transgressions towards us!’

Experts said the language, use of capital letters, poor grammar and spelling mistakes supported the belief that the email was written by a working-class militant from Britain.

The fanatic has raged against Western foreign policy – in particular US air strikes on IS targets – in rants directed at Barack Obama and David Cameron in his beheading videos.

He has referred to recent US attacks, such as the targeting of the Mosul Dam and Haditha Dam in Iraq, to make it clear that the films were shot shortly before their release.

Brutal: Jihadi John is understood to have become radicalised almost overnight and left Britain more than a year ago, where he quickly became a hardened fighter. Above, before executing British hostage David Haines

Jihadi John also appears to be motivated by a desire to fill IS’s coffers with ransom money, demanding £80million from Mr Foley’s family. European governments are believed to have paid millions of euros to free at least nine hostages held with Mr Haines and the Americans.

The militant is said to have called for the release of female Pakistani scientist Aafia Siddiqui, currently held in a US prison, as part of a deal to free Mr Foley.

This afternoon, Mr Henning's devastated wife Barbara described how the family was 'numb with grief'.

Barbara Henning described her husband as a 'decent, caring human being' while her brother described the aid worker's killers as 'scum' who should 'get what's coming to them'.

'It is the news we hoped we would never hear,' she said in a statement issued through the Foreign Office.

'As a family we are devastated by the news of his death. There are few words to describe how we feel at this moment. Myself, Lucy and Adam, and all of Alan's family and friends are numb with grief.'

She went on to praise the people who had supported her throughout Mr Henning's time in captivity - and those who campaigned for his release.

'We always knew that Alan was in the most dangerous of situations but we hoped that he would return home to us. That is not to be.

'Alan was a decent, caring human being. His interest was in the welfare of others. He will be remembered for this and we as a family are extremely proud of him and what he achieved and the people he helped.

'We now need time to come to terms with our loss. We would therefore be grateful if our privacy could be respected at this time.'

Earlier, her brother Colin Livesey said he family had lost a 'great person'.

In an interview with the BBC, he said: 'My heart's just sunk to a level that I never thought it could do.

'I'm just devastated for my sister and two kids for what they've gone through, and it's so hard.

'We're all just saddened knowing that we've lost a great person in our family.'

He described the aid worker's killers as 'scum', adding: 'I just hope and pray they get what's coming to them.

Clues: Experts believe the killer, pictured before heading Steven Sotloff, is aged under 30. He is tall and slender, and appears to be left-handed. He is now one of the most wanted men in the world

'I've just so much hatred for them.'

And he said the government could have done more for his brother-in-law while he was still in captivity, 'when they knew about it, months and months ago'.

This morning, Downing Street revealed that Mr Cameron was meeting with officials from the intelligence agencies, the military and the Foreign Office at his official country residence Chequers to discuss the killing.

Later, he called the slaying 'completely unforgivable' adding that there was 'no level of depravity to which they will not sink'.

'No appeals made any difference,"Mr Cameron told Sky News.

'The murder of Alan Henning is absolutely abhorrent, it is senseless, it is completely unforgivable. Anyone in any doubt about this organisation can now see how truly repulsive it is, and barbaric it is,' he said of IS.

When asked whether he believed the extremists would kill more of their Western hostages, Cameron suggested the only way to stop them was through military action.

'The fact that this was a kind, gentle, compassionate and caring man who had simply gone to help others, the fact they could murder him in the way they did, shows what we are dealing with,' he said.

'This is going to be our struggle now. ... We must do everything we can to defeat this organisation. We must take action against it. We must find those responsible.'

Today, friends, neighbours and former colleagues of the 47-year-old reacted to news of his death with a mixture of shock, disbelief and anger.