He said he did not know if the death was an accident but could not determine without a reasonable doubt that Listro inflicted the injuries.
The child died May 19, 2008, of blunt traumatic head injury.
Listro, a former employee of the state Department of Children and Families, had been accused of killing him by violently shaking him.
The 44-year-old Listro said he died after falling from bed.
