Blackwater, based in North Carolina, has said its guards acted lawfully and in self-defence after their motorcade came under fire in the chaotic incident.
It has co-operated in the investigation.
Most of the shooting
The investigation revealed that two guards did most of the shooting, the US TV channel ABC said.
The guards - US military veterans hired to protect US diplomats overseas - were responding to a car bombing when shooting erupted in a crowded Baghdad intersection.
The Iraqi government has said the guards deliberately killed the 17 civilians; an Iraqi investigation said there was no provocation for the guards to have opened fire.
After the shooting, the Iraqi government wanted to put the contractors under Iraqi legal jurisdiction.
Iraqis also were upset in April when the US state department renewed Blackwater's contract to protect US personnel in Baghdad.