James Foley beheading: Investigators to probe London Isis links as hunt continues for 'Jailer John' the British jihadist

US intelligence sources reportedly keen to speak with former NHS doctor to determine whether he met the killer

The international search to locate the suspected British killer of American journalist James Foley is growing as police and security services continue to gather information on the identity of his captor.

Mr Foley was beheaded in what his killer claimed was in revenge for US airstrikes against Isis to combat their advances across a swathe of northern Iraq. Video footage of Mr Foley’s death released by Isis showed the militant, apparently a British citizen known as “John” threatening the US in a video entitled “A Message to America”.

Isis began making ransom demands of $132.5 million (£80 million) from Mr Foley’s parents last year, which it did not receive. State Department deputy spokeswoman Marie Harf said the US “does not make concessions to terrorists”, saying: “We do not pay ransoms."

The Telegraph reports that intelligence sources are now looking at Razul Islam, 21, who travelled to Syria and is allegedly believed to be fighting with Isis. He is understood to be on a list of suspected British Jihadists whom the security services are comparing with the footage of Mr Foley’s killer.

His brother Shajul Islam, 28, also travelled to Syria and was arrested and charged with kidnapping in 2012 but was released after his trial collapsed.

A second man, Jubayer Chowdhury, 24, was also charged and cleared of kidnapping. The Telegraph understands that investigators are now trying to establish whether the two men from Stratford, east London, may have information about the identity of Mr Foley’s killer.

US intelligence sources are reportedly keen to speak with Mr Islam to determine whether he had ever met the fighter who beheaded Mr Foley.

Shajul Islam has always denied accusations against him, and says he went to Syria to use his medical skills to help treat injured civilians. Members of his family declined to comment when approached by the newspaper.

On Thursday, it emerged that Mr Foley was most likely killed by a gang of three British terrorists known as 'The Beatles', who are guarding Western hostages for Isis near Naqqa in Syria.

Didier François, a French journalist held prisoner for months with Mr Foley also claimed he has a “rough” idea as to the identity of the suspected British killer.

Richard Barrett, who was MI6’s head of counter-terrorism at the time of the 11 September attacks, says he is hopeful the killer could be located and brought back to Britain for trial.

“They will be able to identify him, although there are four or five hundred people who have gone from all over the UK many of them have made their presence there known,” he told BBC Radio 4’s Today programme.

Mr Foley’s family have released the final email sent by Isis just days before Mr Foley’s death, which warned them he would be killed.

The email said: "You do not spare our weak, elderly, women or children so we will not spare yours!

"You 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!"

The GlobalPost, where Mr Foley worked, said it chose to publish the email because it believes “the text offers insight into the motivations and tactics of the Islamic State."

The US continued its air strikes on jihadist positions in Iraq in a clear sign it will not be deterred by Isis’ threats. US officials confirmed that six strikes were carried out near Mosul Dam, which was retaken from Isis militants this week by a combined Kurdish and Iraqi force.