U.S. officials are concerned that the killings may have been committed by Shiite-dominated government security forces in areas already secured by U.S. and Iraqi troops — inside jobs, in effect.
These would be the government security forces built up by the CIA and Steve Casteel, a long-time US intelligence hand? And no one knows who's doing the killing, eh? The US government openly announces its plans, allows it to be reported from time to time, and still no one has any clues out there as to what is happening? The US goes in, secures an area, then leaves its guys in charge, who then go on to kill hundreds of people, and all is a mystery?
US officials are "concerned". I bet they are. I bet they keep themselves up all night, sick with worry.