US authorities have identified the militant known as Jihadi John responsible for the beheading of American journalists James Foley and Steven Sotloff and British aid worker David Haines.
Speaking on Thursday, director of the FBI James Comey said US intelligence services had now positively identified the man – whose videotaped executions last month viscerally brought home the brutality of the Islamic State of Iraq and the Levant, or Isis, to the west.
Mr Comey declined to comment on the name or nationality of the individual.
Read MoreISIS airstrikes: Who are the US's Arab allies?
The masked murderer speaks in videos with a British accent and is widely believed among many intelligence and counter-terrorism analysts to come from London.
A spokesperson for the UK Home Office declined to comment on Mr Comey's remarks. "We don't comment on matters of security," they said.
It would be highly unlikely for the US to have identified the killer and not informed UK authorities, however.
Western intelligence agencies have been racing to identify Jihadi John since the first video was released by Isis showing the execution of Mr Foley in August.
Unmasking the killer has been a particular priority for US spymasters, who have dispatched extra officials and resources to the UK as part of their efforts.
Speaking earlier this week at the UN in New York, British foreign secretary Philip Hammond said authorities were "getting warm" on identifying the killer.
Read More
Mr Hammond admitted last week, however, that British forces do not know where exactly Isis' hostages are now being held. Isis is currently threatening to execute British aid worker Alan Henning.
A US-led commando raid to rescue hostages held by the militants in July failed after they were moved from their original location in northern Syria.
Jihadi John is said to belong to a clique of British Isis fighters in Syria dubbed "The Beatles" who have been assigned responsibility for guarding hostages by Isis commanders.
The small set of jihadis have been in contact with the families of some of the hostages in the UK in the past and are believed to still have links to their own associates and friends in Britain. Communications between the jihadis have been a particular target for British and US electronic eavesdropping agencies.
Read MoreObama condemns Russia, calls for unity against ISIS
Around 500 Britons are said by security officials to have travelled to Syria to fight for radical militant groups, with most joining Isis.
Many, like Jihadi John, have become embroiled in some of the insurgency's most barbaric activities.
On Wednesday, US officials published the names of two Isis fighters it said were Isis "executioners" – Frenchman Salim Benghalem and Albanian Lavdrim Muhaxheri.
In total, 3,000 European citizens have travelled to the region as mujahideen – a fact that is being seen by EU anti-terror officials as one of the gravest security challenges in decades.
More from the Financial Times:
Apple hits back over 'bendgate' furore
Branson:Take as much holiday as you want
Bankers'pay to be cut from Hill's remit
So far only one terror plot linked to radicals from the conflict in Iraq and Syria has succeeded. In May, Frenchman Mehdi Nemmouche is alleged to have killed four individuals in shootings at a Jewish museum in Belgium. Mr Nemmouche is alleged to have previously fought for Isis in Syria.
The EU's top anti-terror chief, Gilles de Kerchove, warned in an interview with the Financial Times in June that further terror attacks against European states and citizens were inevitable.
Dozens of arrests have taken place across Europe, the Middle East, South Asia and Australia in connection with terror plots linked to Syria and Iraq in recent months.