Pentagon investigators believe the CIA has held as many as 100 "ghost" detainees in Iraq without revealing their identities or locations, a much greater number than previously disclosed, a senior Defense Department official told Congress on Thursday.

However, the precise number of undisclosed prisoners and the conditions in which they have been held remain a mystery, said Army Gen. Paul Kern, because CIA officials have refused to cooperate with Pentagon investigators, denying repeated requests for documents and information on the detainees.