The FBI is asking the public to help them identify a masked man with a North American accent seen in a video released by Islamic State last month.
The man's face is obscured by a mask in the video but he alternates between English and Arabic seamlessly in pro-Islamic State announcements intended for a Western audience, the FBI said.
The FBI have released a segment of the 55-minute video in the hope someone may recognise the man.
An FBI spokesperson said: "We’re hoping that someone might recognize this individual and provide us with key pieces of information. No piece of information is too small.”
FBI Director James Comey has said about a dozen Americans are known to be fighting in Syria with IS.