The investigation began in July, months before it was announced that Palin would be the GOP vice presidential candidate.
Last week, the commissioner whose firing prompted the probe told ABC News that he was dismissed because he refused to fire the governor's former brother-in-law, a state trooper.
"I think that my unwillingness to take special action against her former brother-in-law was not well received," Walt Monegan said. The trooper, Mike Wooten, had gone through a messy custody battle with Palin's sister.
Palin has denied any wrongdoing and has retained private lawyers to represent her in the matter. She has said she dismissed Monegan over an honest disagreement over budget priorities.