The three remaining deaths may have been justified as a response to a perceived immediate threat, the FBI decided.
'Deadly force'
One of five guards who opened fire during the attack was responsible for many deaths and became the focus of the investigation, the newspaper reported.
The shootings violated the rules for using deadly force for security contractors in Iraq, the newspaper said, citing unnamed civilian and military officials.
Blackwater insists that its employees were responding to gunfire and had three company vehicles damaged.
Anne Tyrell, a Blackwater spokeswoman, said after the company "supports the stringent accountibility of the industry".
"If it is determined that one person was complicit in the wrongdoing, we would support accountibility in that. The key people in this have not spoken with investigators," Tyrell said.
A previous investigation by the Iraqi government into the shootings, which took place in Baghdad's Nisoor Square, concluded that they were unprevoked and possibly criminal.
The findings are being reviewed by the US justice department.
It is unclear whether there are adequate criminal laws in the US for the guards to be tried.