The state fire marshal's office said Friday that the cause of the fire at Riverside Downs is undetermined, but it does not appear suspicious.
Deputy state fire marshal Chris Pentecost said damage to the barn made it impossible to determine an exact cause. He said the fire originated near an electrical panel.
The pre-dawn blaze Thursday was the second deadly fire this year at the former racetrack, which is outside Henderson, near the Indiana border.
In January, six horses died in a fire that was blamed on a vending machine electrical cord.