In an excerpt of the video released by the FBI, the man, dressed in camouflage and wearing a shoulder holster, speaks in English and Arabic as he stands in front of men he says are Islamic State prisoners.
"We're hoping that someone might recognize this individual and provide us with key pieces of information," Michael Steinbach, assistant director of the FBI's Counterterrorism Division, said in a statement on Tuesday. "No piece of information is too small."
The federal law enforcement agency also requested the public's assistance "in identifying U.S. persons going to fight overseas with terrorist groups or who are returning home from fighting overseas."
The agency said tips could be sent to www.fbi.gov/ISILtips.
Western countries say scores of their citizens have traveled to Syria to fight on behalf of Islamic State, which has captured sections of Iraq and Syria. The group has released videos showing the beheadings of Western hostages in which a fighter with a British accent appears.
The United States has been bombing Islamic State positions in Iraq since August and extended the campaign to Syria in September.
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