New York police are investigating whether rescue workers were responsible for looting shops beneath a burned-out World Trade Center building.
Two men have already been charged with stealing from the rubble, but it is unclear if they were part of the emergency services.
The new looting has been discovered by a New York National Guard infantry unit at the World Trade Center.
It appears to have begun soon after the fires in the upper stories of the building were extinguished, reports the New York Times.
Thieves took watches, designer sunglasses and tried to open at least one cash register. They also broke into a room behind some cash machines but were unable to open the safes.
Guard officials say it is virtually impossible for civilians to reach the shops, which are within a security zone, pitch black and covered with rubble.
But Capt Vincent Heintz said: "There have been people down here trying to steal from a mass grave. They are grave robbers."
Mr Daniel Castleman, chief of the district attorney's investigations division, said two men were arrested on September 13th. One was a former city correction officer who was posing as a police officer. They were charged with stealing two watches at the Tourneau shop.
He said the looting was limited to a few shops in one building. "It could be one guy who was very determined and got his hands on some merchandise. At this point we don't have evidence of who it was, when it was, or precisely what was taken, and the Police Department, understandably, has taken an interest in it".