A former senior CIA officer who was stationed at the base said a combination of agency officers and contractors operated out of the remote outpost with the military and other agencies.
The CIA has not yet commented on or confirmed the deaths.
The congressional official said it was not clear how many of the victims were assigned to the CIA.
The officials spoke on condition of anonymity because they were not authorized to discuss the matter with the media.
U.S. officials said the attack occurred at Forward Operating Base Chapman in Khost province, killing eight American civilians and wounding others.
Before Wednesday's bombing, four CIA operatives were known to have been killed in Afghanistan since the Sept. 11, 2001, terrorist attacks on the United States. CIA officer Micheal "Mike" Spann was killed in a prison uprising in November 2001. An agency officer died in a training exercise in 2003, and two contractors operating out of a CIA base in Shkin were killed in 2003.