Last week, jurors charged Harry Bostick with felony drunken driving, fleeing the scene of an accident and drunken driving leading to a death.
Bostick's truck slammed into the car being driven by a teenager in October -- killing her and injuring her sister. Mississippi Highway Patrol said he was drunk at the time.
Barbour had said that he wasn't aware of Bostick's October DUI arrest when he chose to pardon him.
However, e-mails obtained by CNN show Barbour's staff knew.
Bostick had been convicted three times for drunk driving.
He faces at least 30 years in prison.