British and American intelligence officials have confirmed that they are certain of the identity of the "Jihadi John" executioner fighting for Isis in Syria and are to unmask him "within days".
In both of the terror group's videos showing the deaths of American journalists James Foley and Steven Sotloff, a masked executioner is seen and a British voice is heard.
It is believed that the executioner is one of four British jihadists tasked with looking after hostages in the group's Syrian stronghold of Raqqa. They are known as the "Beatles" by fellow jihadis.
An elite FBI team was flown into the UK last month to investigate the jihadi network which aided "Jihadi John" in his journey to Syria. They are closing in on 12 suspects who are believed to have provided "John" with money and contacts in Syria.
"We are 99.9% certain now as to who 'John' is but investigators have had to tread softly in charting and approaching his wider network in the UK," the source said.
"It has been a co-ordinated effort to track down the support network around the British Islamic State executioner. Our inquiries have given us leads across the country.
FBI travelled to West Midlands
It is reported that the FBI travelled from Scotland Yard to the West Midlands to monitor the movements of the terror affiliates.
"This approach has been successful in helping to collate evidence and possibly pinpoint the target's location in Syria. Familiar names of what can only be described as hardened extremists with a jihadi background have cropped up in this investigation," the source added.
"I expect an official announcement disclosing the executioner's identity will be made in the coming days and it is only a matter of time before we bring him in."
Both UK and US authorities have identified three of the "Beatles" and are now attempting to locate the group before anything happens to British hostage David Haines, shown in the video of Steven Sotloff's death.
The video warned that the British man would be next if the US did not stop its airstrike campaign against the group.
It has been reported that Haines is suffering physical difficulties and health problems in his captivity at the hands of the jihadists. He is believed to have been tortured by jihadists and punished with taser shocks.
The father-of-two, 44, is a former RAF engineer and originally from Perth in Scotland. He was abducted by the jihadists last year at a refugee camp in northern Syria.