"They informed her that she did not drop off the child and they went, proceeded to look for the child inside the daycare," Opgrande said.
"They couldn't find her. That's when they went out to the vehicle, and then discovered her outside in the back of the car. She was deceased."
The grandmother was questioned by police and has since been released.
"She's obviously distraught," he said. "This is a tragic situation and that's why we remind parents all the time. It's hot out and things like this aren't supposed to happen, but they do."
Oklahoma County Sheriff's Office is investigating the child's death at the request of Luther Police.
Opgrande said a report will be forwarded to prosecutors to consider charges.
The names of the infant and the grandmother have not yet been released by police.