It seems to have been a genuine murder, bot not of a CIA agent. From AFP:
A United States official has denied that a CIA agent had been beheaded in Iraq, as claimed by an Islamist web site that broadcast a video purporting to show the decapitation.
“The man depicted in the video is not a CIA official,” the official told AFP, on condition of anonymity.
“No CIA official is missing,” the official said, adding that the Central Intelligence Agency knew the whereabouts of all its agents.
On Wednesday, an Islamist web site showed a videotape which it said was of a US national and CIA agent being beheaded by members of a Islamic militant group in Iraq.
In the poor-quality video, whose authenticity could not be verified, a young Western-looking man is seated on a chair surrounded by five hooded gunmen, one of whom beheads the man using a long knife….
But the gunmen’s words are unintelligible, apart from their cry of “Allah Akbar” (God is great) after the decapitation.