The US says it believes it has identified the British masked man in the videos depicting the beheadings of two American journalists and a British aid worker.
FBI Director James Comey told reporters at the bureau’s headquarters he would not reveal the man’s name or nationality.
Mr Comey did not address whether the US believes the man actually carried out the killings himself. The beheadings are not shown in the videos.
In the three videos, the man – dubbed ‘Jihadi John’ – speaks with a British accent. He holds a long knife and appears to begin cutting the three men, American reporters James Foley and Steven Sotloff and British aid worker David Haines.
Last month British Ambassador Peter Westmacott said his country was close to identifying the Islamic State group militant.