A Pentagon spokesman says he has no information about the fate of American civilian contractor Ronald Schulz.
Lawrence Di Rita also says there's no information about a video on the Web that shows a man being shot in the back of the head.
The video was posted by an extremist group that earlier claimed to have killed Schulz. But the video doesn't show the victim's face, and it's impossible to identify him.
Schulz, a civilian contractor from Alaska, has been identified by the extremist group as a security consultant for the Iraqi Housing Ministry.
Schulz's sister, Julie, says she had no confirmation of her brother's death from US officials.