The FBI believes it has identified the British-accented masked jihadi who appears in the videos showing the beheadings of two American journalists and a British aid worker.

James Comey, the director of the FBI, told reporters at an informal briefing on Thursday that the agency is confident of the identity of the man. The militant gained notoriety after his roles in videos released by Islamic State (Isis), that showed the killing of James Foley, Steven Sotloff and David Haines. Comey reportedly declined to reveal the jihadi’s name or nationality at the briefing.

Philip Hammond, the British foreign secretary, said on Tuesday that UK investigators were “getting warm” on determining the masked militant’s true identity. British security sources have indicated that MI5, the domestic intelligence service, may have been aware of the man before he traveled to Syria to join Isis.

Paul Bresson, an FBI spokesman, declined to say if the agency had shared the identify of the man with its UK counterparts.

Rumors have circulated for weeks as to the real name and background of the jihadi, whose identity is the subject of intense interest from British security officials. He appears in three Isis videos displaying the grisly murders of the US journalists and British aid worker, playing a similar role in each – cloaked in black, brandishing a long knife as the victims kneel on the ground in orange jumpsuits.

He apparently begins cutting the victims, but the full beheadings are not shown in the videos. Comey did not disclose whether the FBI believes he carried out the killings himself.

The jihadi’s apparent London accent prompted a groundswell of concern about UK nationals playing a prominent role in Isis.

A former hostage of the jihadi group told the Guardian in August that the man’s name was John and he was among three British nationals with Isis in its Syrian stronghold of Raqqa. He described the man as intelligent and educated, and a devout believer in radical Islamic teachings.

Isis is holding at least one more UK national hostage, the aid worker Alan Henning, as well as an unknown number of other westerners. Henning’s wife Barbara said on Tuesday she received an audio recording of her husband begging for his life and that a sharia-law court established by Isis had found Henning innocent of spying charges.