The FBI has announced that it believes it has identified the Isis (now known as the Islamic State) executioner known as "Jihadi John".
FBI Director James Comey confirmed the American bureau has pinpointed the foreign jihadist after weeks of transatlantic intelligence efforts but would not be releasing his identity.
The masked executioner is thought to have beheaded US journalists James Foley and Steven Sotloff and British aid worker David Haines in the hostage videos released by the terror group.
He is believed to be one of four British jihadists who joined the group in Syria and are known as "The Beatles". They are tasked with looking after IS hostage's in the group's de-facto capital of Raqqa in north-eastern Syria.
In all three hostage videos, believed to have been filmed in the hills of Raqqa, the militant can be heard speaking in a southern English accent with only his eyes visible to the camera.
British Foreign Secretary Philip Hammond had earlier said attempts to locate the killer of the American and British hostages were "narrowing down the field".
"We are getting warm, we are working on all the leads," he told CNN while in New York for the United Nations summit.
"There's a big investigation and we are getting warm. We're narrowing down the field, but I don't want to say any more at this stage."
The British Ambassador to the US, Peter Westmacott, recently revealed the UK was using sophisticated technology, including voice recognition software, in the hunt for the executioner's identity.
Earlier this month, a group of US senators passed legislation to authorise a $10m (£6.2m) reward for information that would aid the arrest and conviction of the masked militant.
British intelligence officials are also waiting to arrest associates of "Jihadi John" in order to gather more information by monitoring their communications.
The security services have decided against raiding the house of the executioner's family and his friends in order to retrieve as much intelligence as possible.
Security sources have said their priority is to locate the remaining hostages in the terror group's possession and making arrests this soon could jeopardise their safety.