Atlanta (CNN) -- Open a car door on a summer day, and a sauna blast will quickly remind you just how seethingly, sticky hot it can get inside in just a short time. It's suffocating.
For 22-month-old Cooper Harris, strapped all day into a child's seat in his father's SUV, as the sun baked it, it was fatal.
Investigators in Georgia wanted to know how high the temperature climbed in that back seat, so this week they recreated that sauna heat inJustin Ross Harris' silver Hyundai Tucson.
They drove it to the spot where it sat in the sun for seven hours on June 18, the day Cooper died.
They have not released the data yet, but CNN weather experts believe temperatures could have climbed to nearly 140 degrees inside the car.
The National Highway Traffic Safety Authority has corroborated the possibility.
"Even outside temperatures in the 60s can cause a car temperature to rise well above 110° F," the agency said.
The test came in the same week that the Cobb County medical examiner's office said toxicology tests on the boy revealed nothing abnormal, meaning he apparently was not drugged or medicated.
That report and the autopsy report -- which found the child's cause of death "consistent with hyperthermia" and that investigative information "suggests the manner of death is homicide" -- will not be released to the public until the investigation is complete, the office said Thursday.
Cooper's father, Justin Ross Harris, is charged with murder and child cruelty. He has pleaded not guilty.
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