James Comey the FBI director said the man is likely from North America.
Comey would not reveal the name of the masked man who top officials in the UK have previously said is British.
Comey said international partners and the FBI have been able to identify him.
In a number of Islamic State videos where hostages from the West have been beheaded, a militant who is armed, possibly the same person every time, stands next to them delivering statements against the actions by West in Iraq.
James Foley as well as Steven Stoloff, both American journalists, and David Haines an aid worker from Britain were killed in similar brutal fashion. Another citizen of Britain, Alan Henning was threatened with the same in the most recent video.
Islamic State has wreaked havoc while capturing large areas of land in Syria and Iraq and the military of the United States is conducting a sustained bombing campaign with coalition forces in both Syria and Iraq.
The U.S. is worried about the media campaign Islamic State is using which pushes propaganda online through social media encouraging people from across the globe to launch their own attacks in their own homelands.
Earlier in the week, in the latest video, perfect English is spoken by a man who is has a North American accent.
Comey spoke about the latest group the U.S. is worried about. The Khorasan Group has become another target for the U.S. and their airstrikes. The group, which little is known about, was closing in on the execution phase for an attack in the U.S. or Europe.
The group was working on new designs for their bombs so they would be able to be smuggled aboard flights bound for the Europe or the U.S.
The group, said Comey, is at the top of his list of worrisome things since they are a group of experienced and very bad terrorists who operate in a safe haven into which the agency does not have complete visibility.
Because of such limited visibility, we do not know if an attack would come today, tomorrow or when, said the FBI director.