But Comey declined to reveal the man's name or nationality, according to ABC News.
The masked, black clad militant, with a British accent, killed two American journalists -- James Foley and Steven Sotloff, and a British aid worker David Haines -- in videos released by the terrorist group that has controlled large swathes of Syria and Iraq, Xinhua reported citing the report.
FBI declined to reveal the man's name or nationality
In the three clips, the suspect holds a long knife and appears to begin cutting the captives. The militant -- who speaks with a southern English accent -- shows only his eyes in all three videos.
From the footage, the executioner appears to be left-handed and of average height and build.
The FBI director also said over 100 Americans in total had either managed to reach Syria, had been arrested while trying to travel there, or had gone and returned, the report said.
Comey indicated that terrorists in Syria may still be plotting attacks on the West, and there was no sign yet that US airstrikes this week had disrupted the group's plans.